In Busou Renkin, Victor does this, although it's to combat a giant robot, the Buster Baron. It's explained that he used the bodies of all the dead creatures around him to create a giant version of himself around him. When the Baron destroys the giant form, he leaps out and catches them by surprise. Doesn't explain where he got the giant pants from, though.
In the 13th episode of Digimon Adventure, Devimon grows to giant size, eating Ogremon to help himself do this. After beating up the first six Champions this way, he's killed when Angemon uses up all his energy to destroy him.
Also, the DATS Base doesn't turn into a Giant Robot. Tragic, really.
In Dragon Ball, Piccolo Junior did this during his big fight with Goku near the end of the original series. It was Goku who goaded him into doing it, so he can jump down his throat and retrieve the jar holding Kami.
Lord Slug did the same trick in the DBZ movie of the same name. Apparently this is a common power for 'Super Nameks'.
The Saiyans also do this as well, either by the Full Moon or by using a technique to create an artificial moon to transform into a giant monkey called the Oozaru.
Mazinger Z: In episode 12, Baron Ashura used a size-changing ray to turn a tiny robot into a giant Robeast -Bicong O9-. That ray had been invented by Big Bad Dr. Hell, who previously tested it with Ashura himself/herself, briefly transforming it into a giant. Throughout the series, Hell used more Mechanical Beasts could grow their size.
Protagonist (sort of) example from Watchmen: After the Big Bad vaporizes Dr. Manhattan with an intrinsic field remover, Doc comes back huge. Then he shrinks down to normal size again after smashing the building open with one hand.
Dr Manhattan won the Vietnam War this way as well.
In the MarvelGodzilla comics, as the heroic monster battles the intergalactic Mega Monsters (Rhiahn, Triax and Krollar), their alien commanders zap them with an "Energex Ray," doubling the monsters' sizes and making them tower over Godzilla.
Fridge Logic makes the Stay-Puft Marshmallow Man from Ghostbusters one of these. He started out as a cute little cartoon mascot not unlike the Pillsbury Doughboy, but Ray Stantz's choice of him as the form of the Destructor made him gigantic. In fact, his size and mass are the only dangerous things about him.
While not a villain, Jet Jaguar does this to himself in Godzilla vs. Megalon when he realized he had to distract Megalon until Godzilla arrived. The only problem was that Gigan showed up first, and he and Megalon beat the stuffing out of Jet Jaguar...
Also, Minya in both All Monsters Attack and Godzilla Final Wars. In the former movie it was part of a child's fantasy, but in the latter it was presented as a natural part of Minya's evolution.
Averted in Mighty Morphin' Power Rangers: The Movie! The two Ectomorphicon Titans are already giant, and the only "growing" is when Ivan Ooze merges with one of them. (Maligore does make himself grow in the second.)
An unusual literary example is David Eddings's Belgariad. When the final battle between Belgarion and Torak is about to start, both combatants suddenly decide to become huge for no apparent reason.
Explained post fight by Belgarath. Since Torak was so much larger than Belgarion, they both grew to even the fight, as dictated by the powers that be.
A heroic example: Tanu, the potion master from the Fablehaven series does this occasionally.
In the finale of Book 2, the artifact is protected by what appears to be a harmless cat. When you kill it, it is resurrected as a larger cat. This happens eight times, until the cat is a winged, three-headed, three-tailed monstrosity with snakes sprouting out of its back.
The demons in Tales of Kolmar have size-changing powers. When they fight their mortal enemies the Kantri, they grow to match sizes. Some flames are exchanged, but the battles are won or lost through physically grappling, biting, clawing etc.
Super Sentai/Power Rangers. Whenever the Rangers defeated the Monster of the Week, eight times out of ten the Big Bad of the season would resurrect it at giant size for the Rangers to fight in their Zords. Each villain had a different method for doing this; let's just list the Power Rangers ones - Trope NamerRita Repulsa threw her magic staff to the Earth, Lord Zedd threw a magical grenade, the Machine Empire sent out a tiny robot with an enlarging ray... If you care... Divatox fired torpedoes with grow potion, Astronema fired a ray from her base, Power Rangers Lost Galaxy monsters grew on their own or, rarely, from drinking a potion, Jinxer threw growth cards, which he had in addition to monster and mook cards, mutants tore off "DNA patches," Toxica fired magic seeds, Lothor fired a magic scroll, Mesogog used a device that brought down technobabble-laden rain, alien criminals in Power Rangers S.P.D.used their own Zords, Power Rangers Mystic Force had it as one more easily-cast spell, each Power Rangers Operation OverdriveBig Bad had his own grow method, Rinshi Beasts used a necessarily-vaguely-defined martial arts technique, Venjix... uh, the visuals that accompany "Begin the download, now!" are pretty vague, but consensus is 'download' refers to downloading a growth program into the monster, Nighloks naturally have a second "Megamonster" life/state/form that kicks in when they're blown up, Vrak calls his Zombats to attach themselves to the monster to initiate the growth, and Princess Levira fires the Maximizer ray from the Armada's mothership. Whew. (mind you, the monster being destroyed at small size first didn't become the norm until well into the series, and sometimes can't happen - any form of self-activated growth requires the monster to exist in order to do it.)
In Dai Sentai Goggle Five the defeated monsters of the week would get revived and then sent a humongous mecha copy of their own body to pilot.
The first Mecha using Super Sentai, Battle Fever J, also averted this. Starting with episode 5, the monster of the week would call a giant robot duplicate of itself. In the early episodes, Battle Fever would have to defeat the monster while dodging attacks from the robot, then get in their own mecha to defeat it. In later episodes, sometimes the monster called its robot as it died, and sometimes both both battles happened simultaneously.
It's become such a cliche that in Samurai Sentai Shinkenger, the Red Ranger nonchalantly tells the others in the first episode that the monster's going to revive and grow after its defeat. Sure enough, the monster does so, and Shinken Red doesn't even bat an eye before summoning his Humongous Mecha to dispatch it again. Furthermore, the process is automatic. All Ayakashi have two lives: whenever a human-sized Ayakashi dies, it's immediately reincarnated as a giant. Shinken Red knew this already, having fought them longer than the others.
The Shinkenger crossover in Kamen Rider Decade averts this by virtue of Story-Breaker Team-Up, though: When the Ayakashi Chinomanako stole Daiki's Diendriver and transformed into a corrupt version of Kamen Rider Diend, his biology was somehow altered, meaning that it now had only one human-sized life. Really though, this was done mainly because Kamen Riders don't have Humongous Mechanote However, there are Kamen Riders who face giant threats more often than others, though not Once an Episode. Take Kamen Rider J and Kamen Rider Hibiki. The former makes himself grow whereas the latter, in spite of the size of his enemies, doesn't. Kamen Rider Den-O, Kamen Rider Kiva, Kamen Rider Double fight giant monsters semi-regularly, using their Cool Train, dragon castle, bike, respectively. Kamen Rider OOO often doesn't need such things to deal with giant threats, taking out the first with a sword slash in finishing mode. It's too bad we didn't get to see Decade show the Rangers that you don't need giant robots to deal with giant beasties.
Speaking of Decade, Diend does this to Decadenote Kinda; it's not that Decade grows per se, it's that Diend used a Final Form Ride to fuse Decade with the giant Kamen Rider J into a humongous Decade in All Riders vs. Dai-Shocker as the final trump card against King Dark, the last man standing of Dai-Shocker after its leaders all went kablooie.
In Power Rangers Jungle Fury, growing is a martial arts technique. Monsters are able to do it at will. The Rangers, on the other hand, basically make mecha out of their Ki Attacks.
Interesting version with Power Rangers Mystic Force/Mahou Sentai Magiranger and the last set of monsters, the Ten Terrors/the Infershia Pantheon: They were naturally giant sized, and could make themselves human-sized. They'd go back to their natural size for the last round.
Ninpuu Sentai Hurricaneger has THREE ways the Jakanja could make their Chuunin grow to giant size; depending on which of the Jounin the monster of the week was in the service of, Wendinu either had to launch a mystical scroll onto the battlefield, physically hurl a size-changing mask out there, or—perhaps the most interesting of the three ways—summon a missile that transformed into a blank Copy Giant robot that would scan the monster's remains and become a gigantic copy of the monster.
In Mirai Sentai Timeranger and Power Rangers Time Force, criminals convicted of time-travelling crimes are flash-frozen and shrunk down to the size of action figures. When they are revived, they have to wear a special seal in order to maintain their normal size, and breaking this seal results in their Humongous Mecha-necessitating growth.
Kaizoku Sentai Gokaiger lampshades the concept in one episode, where the Big Bad impatiently fires the growth ray early, causing Gokai Green to remark "He grew even though we haven't beaten him yet! Can he do that?!" In another episode, after the monster grows, Gokai Red snarks "We weren't going to skip this part, were we?" Yet another episode has a monster that has the ability to grow as a latent power — as well as shrinking, which lets it get into the team's mecha and wreak havoc.
Gokaiger also averts it in a couple of special instances: the Disc One Final Boss has a self-styled Humongous Mecha, as does the Mad Scientist who invented the growth ray. The Rival has his own supply of humongous monsters that he unleashes when he wants to Troll the Gokaiger even more. And late in the series when the Bigger Bad shows up, growths become less common and finally stop entirely after the Mad Scientist is killed (though we do get some mecha battles because at that point, the invasion fleet has arrived).
Oddly, the Trope Maker for Super Sentai shows was actually Japanese Spider-Man. Yes, that one. When Toei made a Toku adaptation of Spider-Man, they couldn't think of a good spider-themed finishing move, so they had the villain make the monster grow and then Spider-Man would jump in his giant mecha Leopardon to defeat it. It was that kind of show. And yes, it ended up being a predecessor for Toei's own Sentai.
Gosei Sentai Dairanger also has a bit of funny Genre Savvy about growing. Like Morphin' season two, whose monsters it provides, the Dairanger monsters use grow bombs. When Lipstick Songstress (that's Lipsyncher to MMPR fans) gets pissed at having her 'perfect' face scratched, she whips out a grow grenade (clearly, she has no regard for the unwritten rule that you're only supposed to grow when you're losing.) and you can see the Mooks run away before she activates it, not wanting to be squished by her soon-to-be-giant feet! (Not seen in MMPR 'cause the Dairanger Mooks are different.)
Dairanger also defies in an episode where the Monster of the week tries to grow, but is blocked by the heroes by putting the staff to form a cage to prevent him from doing so.
Munchkin has a "Humongous" card which can be played on any monster in combat. When played on the Fungus, it has a greater than usual effect.
Pictured above is the classic "Giant Growth" card, which increases a creature's strength and toughness by 3 each (for reference, an average dragon in the setting would be either a 4/4 or a 5/5). There are also variations on the card, such as Mutagenic Growth (which can be paid with life instead of mana, but it's effect is reduced to +2/+2) and Enlarge (gives +7/+7, but causes enemy creatures to block it)
The use of +1/+1 counters on creatures is seen as growth in size as well; this is used in various mechanics such as Devour, Monstrosity and Evolve (to name a few). There are also variants on the aforementioned "growth" cards that gives +1/+1 counters, although usually in less quantities because of the fact that +1/+1 counters are permanent rather than a temporary boost.
Some Auras giving power and toughness boosts are represented as this as well. The most well known is Eldrazi Conscription. There's also Gigantiform, which can cause a chain reaction of things getting huge.
The RPG Mecha vs. Kaiju is, as the name implies, about mecha pilots fighting monsters. Most are already giant, but it acknowledges the Sentai convention in one sidebar and gives rules for duplicating it in the game.
In Spore, during the Space phase, you can eventually obtain an enlarging beam to make a creature colossal in size.
Yoshi's Island has giant Baby Bowser, who's big enough that if he gets too close, he destroys the entire castle and Yoshi plummets to his death.
In fact, in Yoshi's Island every single boss except two is a normal enemy enlarged by Kamek's magic (The two being Prince Froggy, who remains normal sized while Kamek shrinks Yoshi, and Roger the Potted Ghost, a ghost not belonging to any normal enemy species).
Yoshi's Island DS has similar for a giant adult Bowser made that way, again by Kamek's magic.
Also true for every boss, with the exception of Hector the Reflector, Priscilla the Peckish, Six-Faced Sal (all of whom don't belong to any enemy species), Moltz the Very Goonie, Big Guy the Stilted (who are both naturalKing Mooks in that they were already giant when Kamek brings them in) and Baby Bowser.
A monster unit in Disgaea 4: A Promise Unforgotten can fuse with another monster to increase its size by about three times, which grants them a multitude of benefits. In a boss example, Deathzet does this through her own will for the second phase of her battle.
Speaking of temporary growth, there's also Bowser's Giga Bowser Final Smash.
...and Jigglypuff's as well.
Ratchet & Clank is made of this trope. The boss at the end of the original pilots a giant mecha. Ditto for Deadlocked and Tools of Destruction. Going Commando has the final boss be a suddenly supersized enemy. This trope is also the main idea behind Size Matters.
In Banjo-Tooie, Klungo grows to giant size after consuming his red potion. "Now Klungo sssquasssh!"
In the Touhou fighter Immaterial and Missing Power, final boss Ibuki Suika does this as a function of one of her declared attacks "Second Card [Giant Oni on the Scroll]." Her power is control of densitynote Presumably by increasing volume and increasing density so it only makes sense that she can do this.
in a Touhou imageboard, there was once a discussion over the implications of her abilities, one of them being the possibility that Suika, while in her mist form, could be inhaled; another being that, if she doesn't have a limit to how big she can become, she would be one of the series' fastest characters (despite herself already having impressive agility, according to tengu reporter Aya Shameimaru, who claims to be the fastest character in Gensokyo, the game's setting), with her smallest moves surpassing by far the speed of light; the last of them being, as a joke reply to who would be the most powerful character in the series, one poster mentioned her scale-increasing ability, to which someone else replied saying that "n times zero is still zero"
After each boss battle in Ape Escape 2, you would capture the boss, except for Yellow Monkey, who just ran away. Later, The Big Bad has his scary speech interupted by a banana that fell on his head. Looking up, he sees Yellow Monkey, now gigantic, who you have to fight.
In many of the Mega Man X games, the Big Bad Sigma. Each game ends with you facing off against his multiple forms. His last form is almost always a large robotic body that fills up most of the screen.
This is the resident Monster Clown's whole schtick in Klonoa: Door to Phantomile. He throws something at you, it turns into a giant monster. And later, he does as well.
World of Warcraft has Prince Kael'thas who, after reaching about half health, will start going mad with power, shattering the giant windows behind him and about doubling in size.
Agent Smith actually does this in The Matrix: Path Of Neo. No, really. All of the different Smiths form one giant Smith as the final boss.
Dark Souls has Ornstein and Smough, a Dual Boss and That One Boss. The fight has two phases, and when you kill one of them the other absorbs their fallen companion's power. Killing Smough first will cause Ornstein to grow into a giant.
War, the third Horseman of Apocalypse, grows to fifty feet tall for the second half of his boss battle.
During Super Freakin Parody Rangers, Rita's son Zett orders his monster maker to make it Bigger, to which the monster maker replies "That's what she (Rita) said, to which Zett says "That's what your mother said!" to which the monster make retorts "No, that's what *your* mother said. When the monster grows, Meat the red Ranger replies "Yipes, I didn't expect that. What now?" and Zordon replies "Um, the robots like join together or something. Try that."
Oddly enough, this is used by a hero in Ben 10: Alien Force. One of Ben's alien forms, Humongousaur, can become larger at will.
The original Ben 10 series had this in The Movie, when Ben had to rush across the galaxies to find the Omnitrix's creator, Azimuth, before it self-destructed, tearing the universe apart. Once the threat is neutralized, with Vilgax nearby and ready to kill them, Azimuth turns Ben into an alien bigger than the planet they're on...who Ben immediately christens, "Way Big".
In Turtles Forever, the 2003 Shredder becomes humongazoid for the final battle. It's apparently a feature built into his new body from Krang's technology.
Krang's suit can be seen doing this in an early episode of the 1987 series, as well as in the video game Turtles In Time. He tries it on 2003 Shredder in Turtles Forever too, but doesn't do so well.
Shredder has this done to him by a dark fairy in the Japanese-made Legend of the Supermutants OVA.
In the Series Fauxnale of Dexter's Laboratory, after Dexter and family have the monster on the ropes, Mandark sweeps in with a laser tank so he can finish it off and get all the credit. However, he's too busy gloating to notice that the monster is absorbing the energy and growing even larger, until it picks him up and eats him (He got better).
On Muppet Babies, Bunsen tries to defeat a giant dough monster (played by Piggy) with his beast blaster. Only he used his yeast blaster by mistake, and we all know what yeast does to dough...
Bunsen then counters this by inflating Gonzo to her size with his weirdo inflator. But then the two start falling in love instead.
One What If? episode of Futurama has a giant Bender terrorising New New York, so the Professor uses a growth ray on Zoidberg to get them to fight one another.
The Shushu Rubilax in Wakfu is pretty puny normally (Yugo is taller than him) but each time he gets hit his size increases. Unfortunately, Sadlygrove learns this after he has already rained dozens of blows on Rubilax.
In The Problem Solverz episode "Breakfast Warz", Professor Sugar Fish uses the power of his cereal to become gigantic before the fight with Danny's mom. Because sugar makes you grow big. Crazy big!
Applied liberally in SWAT Kats, especially when Dr. Viper is in the picture.
Subverted in Teen Titans in the episode Mother Mae-Eye; an army of evil gingerbread men combines to become one giant gingerbread man. Beast Boy responds by turning into a T-Rex and biting it in half.