A character suddenly finds their breasts enlarged to often immense levels, via Applied Phlebotinum, magic, science, or otherwise. Mostly happens to flat-chested characters suffering from A-Cup Angst, though women on the other side of the spectrum may have it happen as well. When played for the Fetish Fuel side, the actual growth tends to be as important as the final size of the breasts. It is also frequently played for comedy, but this still ends up providing Fanservice. An Overnight Age-Up for women usually guarantees this will happen.
If the expansion is merely caused by puberty, there's a separate trope for that.
An alternative is Fake Boobs.
A commercial for Brahma features two girls who have their breasts grow as a result of a nearby man blowing up a beach toy.
An advertisement for the racing game Juiced features two teenage boys finding they can enlarge the breasts of a nearby girl and strip her of her clothing by modifying their in-game car.
Anime and Manga
UFO Princess Valkyrie has a scene in the second season after Valkyrie makes a cookie bigger with magic. Just after that Hydra makes a joke about making Akina's breasts bigger. Guess what happens.
The series as a whole features Breast Expansion heavily. Every transformation sequence features prominently a scene of Valkyrie or Hydra inflating their breasts to Gag Boob capacity as they turn into their adult forms.
The second episode of Abenobashi Mahou Shoutengai had Sasshi defeat the final boss by using a magic talisman that Arumi bought, which turned out to be used for breast expansion - the boss's chest swelled up so much it exploded. Yikes.
It should be noted that this was a giant monstrous skeleton in armor.
The opening theme for the first few arcs of NEEDLESS shows Eve having breast expansion after she has her drink.
A breast inflation spell has been used twice in Mahou Sensei Negima!. The results are always a hilarious fail:
In one example, Asuna gets in an argument with some other girls over breast size. To help her along, Negi enhances Asuna's breasts by using a spell to make her swim-suit inflate like a balloon. At first Asuna happily belittles a snooty blonde girl who was making fun of her, but then the spell backfires and she overinflates, causing the chest area of her suit to explode.
In the other Anya and Nodoka attempt to inflate their breasts through sorcery, ends with them humiliating themselves in front of their classmates by overinflating and floating away like balloons.
At the end of the story, Nodoka's breasts have started to grow as a result of being in low gravity.
Sailor Moon featured an odd variation of this. One of the monsters of the week shows up as a flat-chested girl in a swimsuit. She then produces two magical balls and swallows them, and instantly grows boobs of the same size. The action then freezes and a message comes up reading "PLEASE DO NOT TRY THIS AT HOME."
In the newer Dirty Pair series, Dirty Pair Flash, Kei's transformation scene sometimes includes her breasts doubling in size and ripping through her shirt.
School Days: Episode 5. One of the girls uses an inflatable bikini inserts to give herself large breasts.
That's also in the original game, and IIRC the girl who did that was the local flat chested girl Setsuna.
Yuki the Ice Spirit from Master Of Mosquiton gets a transformation sequence where her breasts literally inflate like balloons when she turns from a Token Mini-Moe into a buxom hottie.
In Puni Puni Poemy, one episode sees some of the characters (including the Token Mini-Moe) imagine what they would do with the titular character's magic powers...two of them, including the Token Mini-Moe, imagine them making their breasts larger.
While the girl with Gag Boobs pulls the inverse, imagining a world where she's finally flat and free of back pain.
In Pokémon's episode "Beauty and the Beach", James of Team Rocket wears a breast-expanding bikini top and taunts Misty about his greater size. When the episode was finally aired in the West, the scenes featuring this were cut.
Inverted with May - her breasts (possibly due to being Off Model) go from the size of a grown woman's to mosquito bites by the D/P arc.
An episode of Tokyo Mew Mew involved the girls fighting a pair of birds who inflated their breasts to use as weapons. Specifically they use them as balloons, bounce the main heroine around till she's dizzy, and chasing the boys around.
In Queen's Blade, the character Melona (ironic name) can fire corrosive liquid from her breasts as an attack. When Leina fights her a second time, she has a countermeasure: a special suit of armor that for some reason or another is immune to Melona's acid. Knowing she's outmatched, Leina tricks Melona into attempting an acid attack before dashing up and grabbing Melona in a bear hug. The acid backs up, and inflates Melona's breasts to the point they explode, killing her. Of course, lucky for her fans, it was just the first episode and she's quickly revived by the Big Bad.
Melona is a demonic blob creature in human shape whose body consists of highly acidic jelly, so there is some explanation for how she does this (as well as survive).
Seras Victoria starts at a fairly average cup size and then swells up enormously after she becomes a vampire. Apparently, vampirism comes with some serious breast expansion side effects.
Some pin the apparent expansion to her old police body armour squishing things down. A theory born out when you watch the OVA where she didn't have any body armour and filled out that police shirt quite nicely.
In Cyber Team in Akihabara, Kamome one of the five Magical Girls is ashamed of being flat. So she uses a pump to inflate her chest in the beach episode, and handily attracts a gaggle of boys. Also, all of the girls get pretty damn big when they transform.
In the Transformation-tastic episode of Onegai My Melody, a skinny underdeveloped girl uses (kinda') dark magic to steal other girls' physical features. Aside from stealing one girl's face, and a leggy Tomboy's legs, she also steals a busty blonde's chest size, and some hips. By the time she's done, no one who knew her before even recognized her. Her plan falls apart when she gets greedy, causing the spell to go haywire when she tries to take on too many enhancements at once. This is episode 24 season 1, in case you are "interested".
Grenadier had one episode where the enormously well-endowed main character's flat-chested cohort, Mikan, puts on a pair of big inflatable breasts, ostensibly to show off for her friend it appears.
Jungle de Ikou!'s entire plot centers on a flat chested girl with breast-obsessed friends, who becomes the tall, tanned, massively buxom woman she always wanted to be thanks to magic. Unfortunately it also unleashes a demon bent on destroying mankind... fortunately he's more than happy to just stare at her boobs.
Change 123 - When the main character's split personalities manifest, her appearance changes due to subtle differences in the placement and internal balance of her muscles necessary for their different combat specialties (attack power, speed, and defense). The usual sign that the childish grappling specialist has come out is that her boobs instantly "grow" big enough to snap her bra (or bikini top on occasion). Presumably this is the result of her pectoral muscles pushing them farther out than usual.
Chapter three of the manga series Asklepios has the protagonists trying to convince a village girl to let them operate to remove a tumor on her ovaries that's caused a huge increase of her bust size, but at the same time is killing her. She initially refuses because she thinks her love interest likes big breasts, but eventually accepts the surgery when said love interest says he cares about her and not her bust. After the surgery is over, he even says she looks better flat-chested.
Mahoromatic: Mahoro is fairly insecure about her relatively small bust. She's, therefore, understandably upset in the dream sequence episode where her breast enhancement device works on Minawa, Shikijo, and even the male Suguru, but not on her.
A subversion occurs in Slayers when flat chested Lina Inverse was told of a fountain that makes things grow as a reward for a quest. It turns out the magic spring she was told of would have made her old.
Subversion: Once, Makoto of Hell Teacher Nube stole a bag of magical dust from a benevolent youkai — dust with the power to make anything grow. Among the many scenes of carnage (such as extending Nube's prodigious eyebrows down to his knees, or causing Noriko's cute crown of flowers to completely engulf her,) Miki sprinkled this dust on Kyoko's chest. It inflated... inflated... INFLATED... and just when Miki was horrified and outraged at being surpassed, and the entire classroom stared, two actual melons dropped out of Kyoko's blouse, as the dust had only affected a couple of seeds that had dropped in there during lunch.
A story from the manga Franken Fran featured the titular character (an insane Body Horror plastic surgeon-cum-frankenstein monster) passing out free cosmetic surgery to some kids at a school. Besides odd instances like a couple who wanted to be stitched together, one girl apparently asked for breast implants twice as big as her torso!. The terms "overcompensation" and "something to prove" comes to mind. Hilariously enough, she calls out the other students for the procedures they had done on them, saying that they "lack restraint" and the kicker being that compared to most of the students she's right.
In Black Butler, episode 23, a slightly-endowed Angela tries to tempt Sebastian by showing off her assets, and they actually look bigger than in previous episodes. Possibly justified by the fact that she's not human anyway.
In a chapter of To Love-Ru Darkness, Lala invents a machine that does this. Though all it really did was transfer the size of one's breasts to another for a brief period of time.
In Gou-Dere Bishoujo Nagihara Sora The flat-chested girl Tenka Morooka gets this unwillingly at the end of chapter 3,at first she don't want "them" because they are "heavy" but in the end she decides to keep them.
In the manga ''Okitsune sama de Chu" a cow demon possesses a girl and causes her breasts to grow from flat to about the size of her head. They eventually grow to almost fill a room. Even when the demon is removed, she keeps some of the expansion. It happens a few more times throughout the series.
In an early chapter of the quite obscure manga Benten-sama ni wa Iwanaide (loosely, Don't say that to Benten), this trope is magically invoked by a selfish and petty guy who uses the goddess Benten's power as part of a convoluted plan to publicly humiliate a woman who rejected him. The target's bust begins to slowly and steadily grow (as a convenient digital readout conveniently tracks her gradual progress from 80 to 124 centimeters), and in the meantime she rashly agrees to fill in at the last minute for the role of a princess in a children's play, which naturally involves wearing a very tight-fitting costume. Between the enchantment's progress and her fellow actors' energetic antics, the result is as predictable as it is spectacular.
In Rosario + Vampire, removing Moka's rosary causes her true vampiric self to emerge. In addition to a massive increase in power and a change to her hair and eye colour, the transformation causes Moka's (already fairly noticeable,) breasts and buttocks to enlarge even further.
Yukari has an imagination spot where she pictures herself with breasts bigger than Kurumu, before they grow so big the imagination spot pops. This was in the same episode she had aged into a teenager where the only one bigger than her was the resident succubus.
In one Shin Chan episode, Misae imagines herself with hugely enlarged breasts.
In Seikon No Qwaser, Oribe Mafuyu's breasts grow from one of the smaller girls in the series, to the biggest, thanks to the use of her sword which draws energy from breast milk.
In the manga this also happens, but over time rather than all at once like in the anime.
The Firesign Theatre had a fake advertisement for "Boobie Chew", a gum that would make your breasts larger, on one of their albums. "Boobie Chew will grow on you!"
In Gen 13, one of the side-effects of Caitlin's gen factor activating for the first time includes her cleavage expanding into CLEAVAGE.
Outgunning even Power Girl effortlessly by a factor of HUGE is the short-running (and deliberately-comical) superheroine Mammary Woman, who appeared in the now-defunct British humour comic UT. Initially a parody of Spider-Man, the heroine was a put-upon nerdy schoolgirl with a very flat chest whose life changed forever when, on a visit to an underwear factory, she was bitten by a radioactive bra-strap. Later that night, the inevitable sequence occurs, as in the space of three frames her assets swell from bee-stings, to basketball-sized (bursting her school blouse open in the process), to about the size of three metres across and a triple-Z cup...EACH. She decides to use her superhuman boobs for the power of good, and so becomes the World's bustiest superheroine, Mammary Woman. In one later episode, she even expands them even further in a matter of seconds to about the size of a military hovercraft each, in order to buffer a crashing train and save its crew and passengers...
One of the first foes faced by Young Justice was the Mighty Endowed, an archaeologist named Nina Dowd ("N. Dowd") who was transformed into a top-heavy feline figure too well endowed to support her own weight. In a twist, despite her being fully clothed, her new assets are never clearly seen.
The comic 3 Little Kitties, by Jim Balent, features the villainous Latex Red who takes breast augmentation to a new extreme...her implants are filled with explosives, so if you kill her you blow up too.
Much like the TV series that it spun off (see Western Animation below) the W.I.T.C.H. comic features this. Since the characters are mainly teenaged girls breast size features heavily in their major concerns. Will, the team leader, is a flat chested girl who uses her enhanced Magical Girl form to offset her lack of size, while tomgirl Irma is already the biggest, but tries to make herself bigger anyway out of pure vanity.
In Chaos Campus: Sorority Girls vs Zombies (yes that's the title), one character, Jamie, is a goth-ish poser who complains about feminist issues and "society's obsession with beauty"...who also has volley ball-sized breast implants. She's very short, you see, and so she got increasingly larger and larger implants to compensate, or in her case, overcompensate, for not being "big" anywhere else. Needless to say, her two friends, Brittany and Paige see straight through her.
While originally portrayed as flat-chested, The Authority team member Shen Li-Min, aka "Swift", eventually paid for some very expensive breast implants and likes to boast about it. Given the nature of the series this is likely making fun of other well-endowed comic heroines.
In the parody comic Babewatch, all the female characters have unusually large breasts but one of the characters has inflatable breast implants she can use as a life preserver.
Oddly enough though many comic book females are possessed of the Most Common Superpower, several are also acknowledged to have had their breasts surgically augmented. Among them Outlaw (from Deadpool), Emma Frost (from X-Men), and Cat Grant (from Superman). Jessica Drew is a maybe...depending on how you interpret this◊
The writer for that comic in which that panel took place outright said it when comparing Spider-Woman to Wonder Woman "Spider-Woman has better hair, better costume, Frank Cho implants and a fucked up origin"
As it happened, that turned out to be the exact scene where Drew was replaced by Skrull Queen Veranke.
Mimi Masters in Fred Perry's Ninja High School story "Minor Adjustments", waking up to find that (with a little help from her equally sorcerous sister) she's magically sprouted medicine-ball-sized sweater-stretchers on her previously flat chest overnight.
In Jules Feiffer's Passionella, when Ella the chimney sweep transforms into Passionella the "beautiful glamorous movie star," one change that sticks out is that, while the former shows no perceptible bosom in her shapeless shift, the latter's breasts extend twice as far as her nose.
The aftermath of the Conquest event in Invincible, Eve reforms her body from scratch; after awakening in an intense care unit Mark is happy to see that she's alive and fine, then he notes that her boobs look bigger, Eve remarks that when she rebuilt herself some "improvements" were unconsciously made. That said, her bossom doesn't look any different than usual, Eve was always quite busty since day one, the way Mark immediatly notes how bigger she is now doesn't match the unnoticeable "change" in her actual design.
Later it turned out that Eve was pregnant, which may have had something to do with it.
Little Ego had a story in which Ego found a cream that caused her breasts to grow. Although initially delighted, they continued to expand till they were too big for her to exit the room. Being a Little Ego story, it was All Just a Dream.
Miss Adventure, from Penthouse Comix, transforms from flat-chested to incredibly buxom when she transforms into a superheroine.
Believe it or not, the climax of the 60s British comedy film The Bliss of Mrs. Blossom hangs on a character sabotaging a demonstration of a new kind of inflatable bra causing the bras to inflate out of control.
In the B horror movie Bolt Neck, one character is a tremendously insecure teen girl. She disappears after the monster, a Frankenstein ripoff, shows up and everyone thinks she was killed...turns out she left to get illegal breast implants overseas. At the end she displays her newly inflated chest to the whole school, with just the results she hoped for.
Bruce Almighty tried this on his girlfriend almost as soon as he got the powers of God, despite the fact that the girlfriend was played by Jennifer Aniston (in real life a 36C, according to those who claim to know).
The end of Dude Wheres My Car after the two leads give the alien necklaces to their girlfriends.
Everything You Wanted To Know About Sexnote But Were Afraid To Ask. John Carradine plays a Mad Scientist of the Masters & Johnson's variety. One of his subjects is being fed silicone into her chest 24 hours a day. "She used to be flat chested, now look at her!" When the lab explodes, a 50 foot breast starts terrorizing the countryside.
In the Asian (Chinese?) comedy Twinkle Twinkle Lucky Star, a man becomes able to grant wishes, hence the name. One instance includes a woman who wishes for enormous breasts, but he decides to make her breasts so huge she becomes unable to stand, a la the Mighty Endowed.
In When Evil Calls, one death scene features a nerdy girl who gets inflated from flat to absurdly huge in a few seconds, which she looks rather happy (if shocked) about...till her chest literally explodes a few seconds later.
A similar situation occurs in the movie Leprechaun 3, where a woman wishes for a new, busty body on one of the titular imp's gold coins. But instead her breasts (and hips) grow out of control until they pop.
Similarly, in the horror film Corpses the evil force causing the ruckus in the film causes one of the female character's boobs to swell up till she explodes.
In the Are You Being Served? movie, Grace Brothers have an expanding bra on sale (in case the wearer comes across a rock star), which comes in handy when one of the characters is attacked by a randy terrorist, only to be startled by the sight of their 'breasts' expanding rapidly.
In Jekyll & Hyde,Together Again, Tim Thomerson plays a plastic surgeon who gets distracted while helping a patient get larger breasts and winds up pumping her up to a cartoonishly large size. When he offers to fix her, she refuses, saying her husband is going to love them.
In the Gossip Girl prequel, It Had To Be You, one of the girls becomes insecure that her breasts aren't growing, tries to enlarge her chest artificially using hormone pills. It works at first, but soon backfires spectacularly, as she keeps getting bigger and bigger and eventually gets ''so big'' she becomes a laughingstock.
In "Savage Breasts", part of an anthology called Smart Dragons, Foolish Elves, a woman gets a mysterious exerciser to make her bosom larger...but her enlarged breasts have a mind of their own.
In "Dork Diaries", a rather humorous parody of Diary of a Wimpy Kid, the central heroine punctuates her stories with drawings to explain the narrative. One of which is her fantasy of what she wishes she looked like...complete with larger breasts and an arrow pointing them out specifically, along with other cosmetic enhancements (though, unlike many characters here, they were simply very large instead of comically huge).
In Fatal Beauty by horror novelist William Schoell, a corporate executive who's unhappy with her bust size uses her company's new miracle cosmetic product to increase their size. Unfortunately, the product is made from the secretions of psychic genetically engineered monsters, so it causes horrible problems to all the test subjects. In her case, the tits grow enormously large and develop a mind of their own, turning homicidal and prehensile, and her best friend is forced to kill her to save herself and others.
The title character of Lucy Taylor's short story "The Flesh Artist" is Cyrus Sweet, a sociopathic plastic surgeon who convinces his lovers to let him "improve" them. Evelyn, the small-busted sculptor who grudgingly agrees to go under his knife, receives a gargantuan pair of mongers. She dumps him immediately. Sadly, her problems have just begun. Years pass, and Sweet pines for "Tits" (his charming nickname for Evelyn). He looks her up, gets an invite to her house, and makes several awful discoveries, all related to the leaky sillicone he planted in her chest... and the terrible yet apt revenge she takes on him for doing it.
This is actually similar to an online Breast Expansion short story, about a millionaire who becomes obsessed with a porn star's breast implants and coaxes another girl into getting dangerously oversized implants herself. Oh and the porn star, named "Mountain Tops", is mentioned in the epilogue in a passing, saying her breast implants "exploded" not long after...
Live Action TV
In Charmed episode Hell Hath No Fury, Paige's breasts expand due to using magic for personal gain.
The Collector: In the third episode of the first season, a svelte supermodel rejoices when she finds her breasts swelling to massive size, but expresses grim dismay when the same thing happens to her ass.
Attack of the Show! has a spoof of home exercise equipment and breast enlargement remedies called "EasyCurves".
An episode of Popular had one girl trying to make herself look larger with an overinflated pump-up bra...with LOL-worthy Epic Fail results.
One episode of Everwood featured a cheerleader who had her breasts surgically enlarged to make herself more popular but she soon suffers from complications from the operation. The main character (a doctor) says they have to go, but she's so desperate to look bigger she initially refuses, thinking she'd be humiliated.
An episode of 1000 Ways to Die included the story "Titty-Titty Bang Bang", which featured a woman whose botched boob job started expanding out of control, eventually exploding and killed her. (Note: said story was busted on Mythbusters, but then 1000 Ways To Die doesn't bother with trivial things like credibility.)
An early nineties cop drama, Pacific Blue, featured an episode where teen girls were dying because they were having silicone injected into their breasts to make them bigger, with the help of a disgraced former plastic surgeon. They were financing it by stealing cars. Truth in Television: in the early days of plastic surgery, this method of breast augmentation was somewhat common, and obviously quite deadly, yet it didn't stop anyone from trying. Yes it was that important to them!
Subverted in House: House suggests breast enlargement as a convenient explanation for the scars from a diagnostic surgery when the patient (an Air Force pilot who wants to become an astronaut) insists that the team do nothing that would make NASA's doctors suspect she had health problems. When asked why not breast reduction, he points out that she'd be an unlikely candidate.
That's My Bush!: Bush's Dumb Blonde secretary tries to increase her intelligence by buying memory pills, but she gets mammary pills instead. Flight Of The Valkyries plays during her ultimate bust-out, which also saves her and her coworkers by breaking her free of her ropes.
In Will and Grace when an old crush of Grace's with a fixation on large breasts gets in contact with her she wears a water-filled bra to inflate her breasts with predictable results. When the guy turns out to be a jerk it gives her a unique way to wreak revenge on him however.
An episode of Sabrina the Teenage Witch features main character Sabrina Spellman having her breasts magically enlarged by a photoshop-like program she's trapped inside of. After she gets out of the program, she can't reverse the transformation and Hilarity Ensues.
In an episode of Roseanne, the title character is scheduled to undergo a breast reduction to help with her back problems. She dreams that the doctor, Doogie Howser, actually made them larger. When she complains that the surgery went wrong, he innocently asked, "What's wrong? Not big enough?"
A sketch depicts an harassed young woman at a bus stop suddenly transforming into She-Hulk. The early stages of her transformation memorably show her flat chest (under a T-shirt bearing the legend LUDO) expanding and EXPANDING... until eventually, the word LLANDUDNO is stretched amazingly tight over her ballooning bosoms. Only THEN is a loud ripping sound heard. Disappointingly, when we then see her transformed, while she's undeniably attractive, her balcony just doesn't measure up...
In another sketch, Benny plays a man helping a woman pump a flat tire on her bike. Instead of the tire inflating, her breasts grow. Cue Aside Glance.
While not "real", exactly, one issue of Cracked featured a satirical ad for 'Breast Implant Barbie'—a Barbie doll who has had her breasts surgically augmented. Part of the joke was that she could "leak" and deflate, and would need to be pumped back up again.
In the first Ratchet & Clank game, there is an Easter Egg of this. If you stand in front of the lady who starts the hoverboard race in Blackwater City and then do multiple flips, her breasts will slowly enlarge for every flip.
In Tech Romancer, one character named Polin appears to be a young girl who uses her magical abilities to transform into a tall, buxom teenager. She actually gets into an argument with another character over who has bigger boobs.
Her default stance is actually based on that form. Otherwise, she's leaning forward for little reason.
In Spellcasting 301, one of the challenges is a wet t-shirt contest. One of the two things you need to do in order to win is to recruit a very flat chested girl and cast the UPPSSY (reverse condition) spell, which makes her improbably stacked.
In the first game, there's also a spell to explicitly increase bust size... but rather than using it to enlarge breasts, you use it on a stone bust of the school's founder, making it bigger so you can climb into the school's stacks.
Inverted in the intro of Dragon Age II. During said intro, Bethany, the Player Character's sister, has huge breasts as does a female Hawke. Once Cassandra tells Varric to stop exaggerating and tell the story how it really happened, and the game starts for real, they're suddenly much more reasonably sized.
Some of his older, now cancelled comics displayed this theme more blatantly, before he decided he wanted to focus more on the story and charecters:
In (the original) Pulse, Annie Chang, a former flat chested girl gets superpowers through a Freak Lab Accident and as side effect experiences breasts that grew by noticeable amounts throughout the series. Eventually, her partner makes up a story to the press that she's a robot and the breasts are actually missiles.
(co-written by Wolfman1066) Pe EV Eed dealt with a down on luck main character, named Eve. Things changed when she finds and becomes the master of a female genie. As with most comedies involving genies Hilarity Ensues and through the course of the series she goes through breast expansion a few times, ironically not once did she wish for bigger breasts. The effect would wear off each time and she would have returned to normal by the next story.
There's also My Big Life. About a reporter who's writing a column about the trend of women getting giant breasts, she does this by getting gargantuan breasts herself and reporting on her experience for a full year. Fortunately, because the series takes place in the future, there's technology available to turn her huge breasts to nothing more than an inconvenience. Any change was implied, her natural body was never shown, nor the process of her changing.
His most blatant example of this trope was a Final Fantasy fan comic called Missing Materia. Yuffie gets her hands on a rare materia that causes growth and she abuses often, sometimes just to annoy people, all the while Materia from all over the world is mysteriously vanishing. By the end of the comic nearly every woman effected has breasts so big they can't move.
In one storyline from Sore Thumbs, normally flat-chested Harmony gets breast implants when the busty heroine of the series disappears and she feels she has to pick up the slack. She later had them removed because her friend returned...and because she initially had trouble standing upright under the weight.
In a milder example, Chloe of Eerie Cuties demonstrated that Succubus puberty was rather abrupt at times.
A cause of concern for fifteen-year-old Atsali, because it's part of the image that Sirens have as sexual predators.
Any woman who has sex with Sir Thane in Bloomin Faeries will grow large breasts, in addition to becoming beautiful and generally sexier overall.
The web comic Supermegatopia (a furry world with many superbeings) has their BE mascot in the subtly-named Buxom Gal. Her origin involves a breast expansion experiment carried out by aliens. Afterwards, she is left with standard superpowers (superstrength, flight, ability to fire power bolts), but discovers that - if she does not use her powers - her breasts (which "store" the energy for these powers) expand to incredible size, with incredible speed.
Strangely, El Goonish Shive doesn't use this quite as much as one would expect - the closest example being the infamous Female Variant Five transformation beam, which increases hair length and bust size among other effects. Also outright subverted in an Omake comic, where Ellen attempts to invoke her "Wonder Twin Powers"...and fails to increase her bust.
A defunct webcomic by an artist named "Jae", titled simply ''Amy'', focuses on the eponymous young woman who undergoes a breast augmentation procedure because, as she puts it, "I'm 18 years old, and FLAT CHESTED!" The doctor is so concerned with looking at her ass that he botches the thing, and she's left with breasts rapidly increasing in size...unable to get back in contact with him to fix her new knockers, she has to face the grim outlook of breasts twice as big as her head and still growing.
In Jix, Lauren (the only human female character) has had this done to her off-screen because Lamerix (the techy character) often uses her as a human guinea pig. Lamerix is testing a breast enhancer device so that she can market it to her alien species. (She figures the male Ambis, her species, would enjoy it and it may make nursing easier.) The real reason, or so the author claims, is to make the female Ambis look more feminine since they were originally designed flat-chested.
In Held Within Strip #13 Susie tells her girlfriend Jenny that she first noticed her when Jenny's breathing made it look like her breasts were expanding. Later on in Strip #21, when Jenny starts changing into a genie, her breasts really do expand.
There are several pay-for-view webcomics that have Breast Expansion as the major theme, often going under a banner like "BE Story Club", with several different series encompassing different methods and sizes.
In the "Amplified Bible" episode of the Foamy the Squirrel animated web series, Foamy casts a spell from the book Bigger Boobs Through Witchcraft on his pet human, Germaine. Her chest swells enormously — a fact neither of them realizes until the next episode where they are so big she can't get sit up, and they still kept growing. The spell seems to have lasting effects, as Germaine's breasts are noticeably larger in the later Foamy episodes than they had originally been.
In the episode "A change of Heart" after her breast reduction surgery Germaine expresses an interest in A magic ring that lets her control her breast size. Foamy points out it defeats the purpose of her reduction, to which Germaine replies she only hated them because she couldn't turn them off when they didn't benefit her, having grown to rely on them for big tips and discounts from horny men. She later plays with the ring in the bath, making her breasts bigger and smaller via putting on the ring, and at one point makes them so big that her stalker's head explodes at the sight.
Since the Whateley Universe features a significant number of mutants whose "body image template" endows them with superhumanly good looks, it's probably a safe bet that quite a few mutant teenage girls are going through this while changing. There's also a humorous, if slightly squicky, example in one of the Jade stories where one of Jade's autonomous telekinetic constructs gets herself equipped with artificial inflatable breasts operated by remote control to help her serve as an icebreaker at a party. Needless to say, Hilarity Ensues.
Rather less humorous (for Phase) was when Belphegor sabotaged a serum made by Jobe Wilkins with the intention of reversing Phase's Gender Bender status. Unfortunately, not only did it fail, it failed spectacularly: his breasts, rather than shrinking, grew to Gag Boobs proportions, and continued growing for several hours while Jobe worked to reverse the effect.
In the Paradise setting, humans are being randomly, permanently changed into Funny Animals (and sometimes gender-changed at the same time). In keeping with the Wish Fulfillment inherent in the setting, female Changed receive an extra helping in the endowment department over what they had as humansand those who used to be male humans get even more.
The content of the Breast Expansion Archive is just what you'd expect from the site's name. Its sister site, the Breast Expansion Story Club (subscription required) is more of the same, but in story form, rather than images. A small sample of what's offered follows:
The Best Cheerleader: a cheerleader steals a breast enhancement formula from a rival to try and give herself huge boobs but it backfires.
New U is about a girl who opts to grow larger and more buxom through designer drugs because she's insecure around her taller, more buxom sister, who got them through a similar procedure.
Hollywood Hills is about an aging movie star with "decreased assets" who undergoes a procedure to try and regain her formerly impressive chest, and finds herself gaining more than what she had.
Til She Gets Her Way has a normally flat and short girl whose tired of being pushed around gain the power to grow in various ways until she gets what she wants, but can't stop the growth until she gets it.
Accident has some girls exposed to chemicals start growing to ridiculous sizes. One, being a an A-cup, is ecstatic. The others, being huge already, are not.
The Goddess' Fun Shop is about a jealous, flat-chested girl who is so upset her sister (who is younger) has huge Boobs that she goes to extreme lengths to get her own. Naturally this doesn't end well.
... Keeps the Autodoc Away deals with the physiological issues caused by outsized attributes. Nothing quite like nearly smothering on your own breasts because you got turned around the wrong direction in Low G...
Welcome To Chastity has a flat chested girl move to a mysterious town were all women have breasts from huge to 'makes up most of your body weight', and those who don't will eventually find themselves growing bigger and bigger.
Stranger than fiction releases a nerdy secondary charecter back into his own story with the power to bend reality at his whim, and finds himself turning every female he comes across into an overly buxom babe who 'at least' wouldn't say no to whats in his pants.
Mordred from Fate Nuovo Guerra got this as part of her Gender Bender package from the Holy Grail. note To clarify, due to being a clone, he was inserted into a Servant body based on his mother Arturia, only a couple of sizes bigger.
In the Brace Face episode "Busted", central character Sharon buys a pump to make herself bigger. She ends up overdoing it, with humiliating results. First getting laughed at by the same people she was trying to impress, and then her boobs...pop.
In the American Dad! episode "Tearjerker," all the gadgets created by S are all just different vehicles for Breast Expansion technology. As befits a James Bond parody, the fate of the world eventually depends on Stan making a woman's breasts triple in size.
In another episode, Stan gives Steve "male" hormone pills in an attempt to get him in with the Jock crowd. Unfortunately, the wrong pills were used and Steve grows breasts. As a result, he still gets in with the Jock crowd. The episode ends with Stan also taking the pills and showing Steve that adolescent males only respond to one thing.
Pretty much that exact same plot happened 2 years earlier on Hey Joel. Just replace hormones with bulk up powder and jocks with Bill Wyman.
Peter asks Jesus to give Lois huge breasts and in the next scene Lois' boobs grow to absolutely enormous size.
Another episode had Lois stop breast-feeding Stewie, and her breasts got so big she was popping buttons off her shirt— just in time to save Quagmire from a bad marriage by reminding him just how much he'd be losing....
Peter remembers how much he misses Lois, and he draws a quick doodle of her on a napkin. Then, he erases her chest and redraws it by several sizes.
On the series the comic book is based, W.I.T.C.H., the characters transform into Guardians granting them wings, power, and a temporary instant puberty growth - which includes growing larger breasts. The typical fan-service associated with this trope is Downplayed but the difference in breast size is sometimes addressed or used as a plot device.
In another sketch, The Bionicwoman is in an accident and needs repair. The Six Million Dollar Man gets the scientists not only to fix her but upgrade her... into a literal sex machine. Including giving her enormous breast implants. Needless to say she isn't pleased and beats the men... with themselves
On an episode of 101 Dalmatians the villainess, Cruela De Vil, tries to seduce Roger by putting on a blonde wig and using inflatable fake breasts. Not only doesn't it work she ends up bouncing so much she literally bounces away. This was a children's show mind you...
One episode of Duckman involves a brief trip to a plastic surgeon, where we see a woman inflating her breasts with a compressed air pump until she explodes. Another featured a short gag about a ray that makes breasts triple in size, as a background gag.
Happens to Marge on The Simpsons, who protests to the doctor as seen in the quote above.
Doctor (bored): "Yes, I'm sure your husband will be furious."
In the South Park episode "Bebe's Boobs Destroy Society", Wendy gets jealous of Bebe's boobs because they've captured all the boys' attentions, and goes to get hers augmented... Only to find when she returns to school that the boys have gotten over their fascination with them, thus leaving her with ridiculously large breasts. Especially considering she's roughly eight or nine.
Ugly Americans: Callie, when she comes into contact with a demon-baby in the third episode.
This is a popular fetish topic on webcams, particularly those involving female bodybuilders. The general idea is the bodybuilder (often one with breast implants) puts on a shirt that's several sizes too small and basically uses their muscles to expand their chest, creating the illusion of expansion. A subtrope of this involves female bodybuilders simulating morphing into She-Hulk by covering themselves in green body paint and doing this.
For those who lack the, um, resources to do the above, a variation involves using an air pump and balloons (or other air-filled item) to simulate the experience.