Guess the size of a woman's bust is a measuring stick for how strong they are, huh?"
—Rei Hiroe, noticing this trope's prevalence in his own series
The tendency for the strongest physical female fighter in a group or cast to also have the largest breasts. Some of the reasons for this trope are discussed on the Analysis page.
A physically strong female without Boobs of Steel is most likely a Cute Bruiser. Compare and contrast with Most Common Superpower and Gag Boobs. See also Amazonian Beauty.
Note that this trope is not common enough to be averted. Unless it is lampshaded, it's just an ignored trope. Also note that in general, this is limited to one or two examples per work.
Examples
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Anime and Manga
Cattleya from Queen's Blade sports a bust far larger than the rest of the cast and as far as raw strength goes, she's far stronger than the rest of the cast as well. She's also very muscular, a true Amazonian Beauty.
Sailor Jupiter of Sailor Moon is physically the strongest of the sailor soldiers and has the largest breasts.
Tsunade from Naruto. She's physically the strongest character introduced in the series yet, and can beat the snot out of guys her own age and relative skill level. With ease. And in the fight with Kabuto, at least in the manga, she recovers fast from an attack that cut off her air, and he wonders how that happened, and focuses his gaze at her chest.
Air Gear has Rika who, out of the Noyamano sisters, is the most physically capable and the most skilled Storm Rider. She's also a professional wrestler.
Saber Marionette J, with Boisterous Bruiser Bloodberry being the bustiest of the three female protagonists, and recon specialist Cherry is the complete opposite.
Cool Big Sis Julia from Rave Master is the sole female melee fighter in the group and also has the largest breasts.
In Outlaw Star, the female with the most physical strength is the obviously well endowed Aisha.
Twilight Suzuka is probably the only female character who's physically formidable and doesn't fit this trope. It's most notable in the "Strongest Woman In the Universe" episode, with every female combatant shown being close to the same cup size as Aisha.
This is likely because Suzuka relies more on Ki Attacks and technique; she's more a Samurai. Aisha on the other hand is not only physically strong but in a pinch can turn into a tiger (she's actually a Ctarl-Ctarl, a race of werecats).
Satella Harvenheit from Chrono Crusade has the most physical (and for a while, spiritual) strength of all the women, and makes her Boobs of Steel very clear to anyone with eyes.
In the Full Metal Panic universe, compare the athletic prowess of Mao, Kaname, and Tessa. And then compare their figures, as demonstrated during their considerable time nude together.
Boa Hancock of One Piece has the largest breasts of perhaps the entire cast. Therefore, she obviously kicks the most ass.
Sakakikido Tsukiko, the main character of Samurai High School and the bustiest. She's easily the strongest person in her school, and her goal is to be supreme commander. Unfortunately, she has to switch places with her twin brother as there are gender restrictions.
Elenore Baker and Madlax from Madlax are quite well endowed; one being theNinja Maid the other being able to destroy armies.
Kallen/Karen of Code Geass. Undoubtedly, she is the most endowed, and she's easily the most physically active member of the Black Knights, being their elite mech pilot. In fact, she's pretty much the only woman in their ranks who's involved in any actual fighting.
She's physically stronger than the vast majority of men in the series. Her superhuman strength isn't on the same level as Suzaku or cyborg Jeremiah, but she's not that far behind him either.
In Sekirei, the most powerful Sekirei tend to be the bustiest, though this isn't a hard and fast rule, with characters such as Kuu, Benitsubasa and Karasuba being extremely powerful, but also modestly endowed. Sekirei also appear to place great importance on breast size and treat it as Serious Business.
In Heaven's Lost Property, Astrea, the Angeloid with the biggest bust, is also the best fighter of the Angeloids even when compared to Ikaros who is slightly smaller. Nymph is an A-Cup therefore she has no fighting ability.
In the swimsuit special for Black Lagoon, Rei Hiroe described this trope and commented on how most of his characters adhered to it — on Roberta's page, he wrote "Her bust is huge, so you can imagine just how powerful she is."
Gangsta: Ginger the Twilight has breasts big enough that they have their own sound effects, along with skills frightening enough to make her an elite member of the Twilight mercenary guild.
In Puella Magi Madoka Magica, Mami Tomoe has the largest bust of the females in the show. So much so, that when she is first introduced, she kicks every witch's ass. Sayaka, while no match for Mami, is also well-developed for her age and seems like she's much tougher than Madoka.
Higurashi no Naku Koro ni: Out of the female cast, physical prowess increases with 'size'; Rika and Satoko are both the youngest (and the least developed); they rely on speed/evasion and traps (respectively) in fights. Rena is a Cute Bruiser at best, who relies on her cleaver to defeat enemies. Meanwhile, Mion (who boarders on Gag Boobs, and is the most frequent victim of Male Gaze) is shown defeating the same enemies with her bare fists with relative ease.
This is justified, however, by the fact that Mion is the eldest except Shion of the cast altogether (she is a few months older than Keiichi). She is also a Mafia Princess, so it's more than likely she's undergone self-defense training.
Comic Books
Astro City takes this to its logical extreme with Beautie, a human-sized robot Barbie doll whose proportions border on freakish at that size: "My skin is ferro-styrene over an omnitanium frame. My breasts and buttocks are rigid. And I have no genitalia." She's also one of the strongest members of the Honor Guard...
In the comic Chaos Campus, pink-haired goth(ish) Jamie is the strongest character, and beats monsters to death with a wrench as a matter of course. So she also has breasts bigger than her head, thanks to enormous breast implants. She's described as "Elly Ripley with silicone."
In Transmetropolitan, Yelena (who is the less-action-oriented of Spider's Filthy Assistants) says she'd have to be hit in the back with two cruise missiles to fill out a dress the way Channon (whose specific responsibility is bodyguarding) does.
DC Comics' Power Girl is famously busty (which has been lampshaded multiple times before), and as a Kryptonian, she's got all the powers of Superman, making her one of the strongest female characters in the DC universe.
Marvel Comics' She-Hulk is confirmed to be the bustiest superheroine in the Marvel Universe and as far as physical strongest goes she's one of the strongest characters in the universe, frequently playing the role of The Big Guy on most of her teams.
Film
In the film Dungeons And Dragons Wrath Of The Dragon God, the strongest warrior is Lux, the tall, busty barbarian. The other two significant female characters are both waspishly slender spellcasters.
In The Dictator, Etra, the "virgin guard" with the enormous Gag Boobs, is also the most physically aggressive, and she uses said gag boobs as weapons.
Live Action TV
Buffy the Vampire Slayer: Faith plays it straight, doling out all kinds of whoop-ass while being one of the bustiest girls on the show.
At least one of Sarah Michelle Gellar's fighting doubles was noticeably better endowed, leading Whedon to note in commentary: "Oh, looks like Buffy's got her fightin' boobs on!"
Tifa is the fighter in Final Fantasy VII, most reliant on the strength of her muscles.
Aqua is hands down the greatest lady fighter in the entire Kingdom Hearts saga, and is the most powerful mage of both her own trio formed of Terra and Ventus, and of later playable characters, such as Sora and Riku. She also gets noticeable attention trained on her bust, like the frequent zooming on on her breasts and villain Hades's ogling at her in her travels.
Even by Dead or Alive standards, Tina Armstrong's boobs are huge, the largest of the ladies at 95cm. And being a wrestler, she is the physically strongest.
Lamia Loveless of Super Robot Wars. She's probably the female character with the biggest boobs between all other Banpresto Originals thus far, and physically the strongest, despite not using her strength too much (being a mecha pilot and all) because... well... she's an android.
Aschen Brodel and Kaguya Nanbu from Endless Frontier both count. Aschen is a very powerful android but she's still beaten in the chest department by Kaguya "Dairy Cow Princess" Nanbu, who fights with a sword that's almost as big as her.
The King of Fighters has Shermie, Angel, Blue Mary, and Vice (who are grappler/power characters) who all have larger bust measurements than Mai Shiranui. Also, in the first KOF Maximum Impact, Lien Neville (no official 3-sizes given) actually has the largest in-game breasts and is fittingly, the strongest female character. The final boss from the second Maximum Impact game, Jivatma, has "another model" that looks female and has probably the largest bust in the game. There's also Vanessa, introduced in The King of Fighters '99, who is notably busty, bounces quite a bit, and is a purely physical brawler.
Ayla the cavewoman from Chrono Trigger has the greatest strength of any party member. She can do more damage with her bare hands than Crono can with his Infinity+1 Sword. And the more you Level Grind them, the larger the strength gap between Ayla and everybody else gets. As Spekkio puts it, she might not get to use magic, but she makes up for it with other assets.
Judith, who is easily the best stacked of the three female PCs in Tales Of Vesperia and is also the only melee fighter among them.
In Tales Of Hearts Innes Lorenz has an even larger bust and is also the token Mighty Glacier character. And she can also lift objects way more heavy than she is, with ease. It's also lampshaded repeatedly, as first the Boobs of Steel and her massive axe blade are the subject of an Innocent Innuendo skit. Later in another skit, Beryl theorizes about Innes' super strength, and the player is allowed to choose her guess: "It comes from her breasts!" or "Innes is a Mechanoid!"
Vanessa of Virtua Fighter not only has the biggest breasts of the female characters, but she's almost a Mighty Glacier.
Marilyn Sue of Akatsuki Blitzkampf not only hits harder than any other female character, but also happens to be both stacked like pancakes and braless, all the better to Gainax with.
The first one has Ronnie Bell, who is called "the giant woman" for a damn good reason (she's at least seven feet tall and might actually be half-giant), and whose chest, due in part to her sheer size, is noticeably the largest of the entire cast. And she's a monstrous bruiser who fights with her fists.
Suikoden II has Oulan, who is by far the most stacked female in the game (she noticeably bounces in her battle animation) and one of the top five tanks in the game as well as a bruiser who can dish out absurd amounts of damage repeatedly if properly set up. She's a bodyguard, and does her job barehanded.
Suikoden V has Nikea, a Big Eater martial artist who keeps up the tradition of dealing tons of damage with just her fists. Also notable is Kisara, an Action Mom who passed on a great deal of 'talent' to her daughter, Lun.
In the Battle Team Lakers series, Natsume is consistently the strongest (physically) of the eponymous team. And despite being a series of H-games, she's the only one with notably large breasts. The others have "large-but-within-reason" busts typical of Silver Age comicbook heroines.
The recent Wii game Muramasa: The Demon Blade has this in the form of Raijin, a hugely muscular woman who is more than happy to squish (or zap) Momohime Jinkuro. Interestingly enough, she has a "cute girl" sitting posture if you knock her down, and an implied thing for her co-god Fujin (who appears as a tiny green imp).
In the side-scrolling beat'em up Knights of Valour the average looking female mooks of the forest respond to the genre's stereotype: they use hit and run tactics, attack mostly using jump kicks, and are quite frail. In later stages of the game, mooks with larger breasts prove to be substantially tougher.
In Disgaea, all of the "human" recruitable women are flat chested, except for the Magic Knights, Kunoichis, Cheerleaders, and Gunslingers. The "monster" recruitable women (Succubi and Nekomatas) are very big-chested, and the Nekomatas are straight up brawlers, utilizing the highest counter attack rate and damage. For storyline characters, Jennifer in the first game specializes in fist-type weapons and easily has the largest breasts in the cast.
Other Nippon Ichi examples: Prier from La Pucelle has the largest breasts in that game and is probably the strongest physical fighter in the main cast, easily kicking down a steel door in one cutscene.
Salome of Makai Kingdom is the most physically-oriented of the female cast members and is also the best-endowed.
Zola in Blue Dragon certainly qualifies, more so with her choice of — or perhaps it would be more correct to say lack of — undergarments.
In Sonic the Hedgehog, boob size seems to be proportionate to how Bad Ass you are as a woman. There's Cream, with none at all, who is very weak. Then there's Amy Rose, who has a gigantic hammer and pounds people with it. Then there's Rouge, who kick-boxes and has not only got large breasts (for a kid's game) but also displays actual cleavage. Though one has to take into account that Cream is a child and Amy is a pre-teen while Rouge is a government agent.
In Phantasy Star Online, it's possible to change the size of the female characters' breasts. The HUnewearl, a melee specialist, get the biggest growth.
Radiant Historia features Raynie, the bustiest of your three female members and by far the sturdiest with a moveset focused on dealing large amounts of damage directly to a target. Compare to the more petite and incredibly fragile Eruca, who focuses on status effects and weak, but hard to resist non-elemental spells; and Token Mini Moe Aht, whose traps have a higher damage output, but require other party members to knock enemies into them.
The Tellius continuity brings us Lucia and, as of Radiant Dawn, Mia. Both are Action Girls with very sizable busts. In-universe both have reputations as highly competent fighters. There's also Nailah, who's the bustiest of the playable female characters and is also one of the most powerful characters in the game, and has an in-universe reputation to match.
The Pirate captain Eiliff Thunderfist from Drakensang is the one of the strongest fighters in the game outside of the main party and has huge boobs. Partially justified as she's quite bigger than other characters and not exactly attractive. Last but not least, The Amazons tends to be this.
Lita Halford from Brütal Legend is the most busty character in the game, which is saying a lot considering that all female characters in the game are large-breasted. She's also a very skilled warrior who fights with a large polearm. At one point of the game she easily defeats the less-endowed Ophelia in single combat.
In Seitenshi Yumiel, Yumi's breasts grow when she transforms into the eponymous Yumiel.
In The Elder Scrolls V Skyrim, the CharGen "weight" slider more is more accurately described as a muscle mass/body frame slider, and for female characters, it's also a breast size slider.
Amidst the "three stooges" of NOL Academy from BlazBlue, Makoto Nanaya is arguably the most stacked, and also freakishly strong. On the whole scale, Litchi Faye-Ling has the biggest boobs and not only she has the biggest, most damaging Astral Heats, story-wise shows that under certain circumstances, she is able to overwhelm a gigantic cyborg like Tager.
In Dragon Quest V, the voluptuous Deborah has the highest physical strength of all the Hero's potential wives and can equip some of the heaviest weapons.
Kae from the SUGURI series of doujin shoot 'em ups is not only known for having a hot, curvacious and well-endowed body, but she's also a fieryfist-fighter.
In the upcoming Code of Princess, the first three playable characters are the BifauxnenAli, the moderately-endowed necromancer Zozo, and the queen-sized Solange. Guess which one wields the 7-foot BFS?
In Hyperdimension Neptunia V, Yellow Heart fights with kicks and clawed punches, and her skills have the most "brute force" feeling of the CPUs. Lots of hard blunt hits and explosions. She also has the stockiest build.
In the indie Beat 'em Up game, BANZAI PECAN: The Last Hope for the Young Century, French Vanilla is arguably the most bustiest succubi of the Sexilicous Seven and she's also a wrestler. The game's titular heroine, PecanApple, is also stacked and she pulls no punches in delivering a beatdown to protect the peace of the world and her boyfriend, though Pecan is nowhere as busty as half the villains in the game.
Yuka in Variable Geo is one of the strongest fighters in the game, and has the largest bust of the fighters.
Visual Novels
Arcueid of Tsukihime has the biggest bust out of the main cast, and is by far the strongest character in the game, both in terms of physical strength and overall combat ability.
Among the cast of Fate/stay night, Leysritt is the biggest among all of the female characters and can wield a giant halberd with ease. She never actually fights in the Visual Novel, but according to notes she's actually physically stronger than most of the Servants.
In Little Busters, Kurugaya is by far the strongest and most Action Girl-y of the female characters and also has by far the largest bust size.
Webcomics and Other Web Originals
The Blonde Marvel has enormous breasts to match her Big Beautiful Woman physique, and has super-strength to match. She once fought a woman with even larger breasts, and was losing the fight until she used her opponent's magic cigarettes to enlarge her own breasts. She then won.
In Crossworlds, the biggest boobs belong to Narhm, who also happens to be 'bout 7 feet tall (counting the horns) and muscled like a bodybuilder, thanks to her day job as a blacksmith. In combat, she wields a huge, two-handed warhammer with devastating effect.
Alice apparently has this boob based power according to this post of Loserz.
Buxom Gal, of course. Her cup size varies in direct proportion to how much energy she's absorbed.
Adharia Kuvoe is heavily implied to have the largest breasts of the female Last Res0rt cast. The fact that she's also a lion-esque centaur only justifies this further, because she's already one of the larger cast members, period.
Damali in Love/Curse is by far the biggest, strongest and most buxom of the main the characters.
Pella from Looking for Group has a somewhat credible physique; being muscular but not excessively ripped allows her ample tracts of land to be shown to their best advantage. Just don't put your hand on her shoulder.
In Sluggy Freelance, Oasis and her Blue Oni clone-ish counterpart Kusari have the largest breasts of almost all the female cast, and are also superhumanly capable fighters.
Whateley Universe examples: Tennyo is the strongest girl on Team Kimba, and (having the shape of an older girl) has the biggest breasts. Compiler is the strongest member of the Whitman Lit Chix, and has the biggest breasts, since she used nanotechnology to improve her body and overdid it.
While most of the women of Girl Genius are rather well-endowed, the strongest and most skilled seem to have the biggest busts, like Pirate Queen Bangladesh DuPree and Zeetha.
In the WatchmenRule 63 community, (yes, there is one) Edy and Joan are usually portrayed as being quite busty, where as Wanda and Adriene are depicted as flatchested. Danielle also tends to be largechested, but that has more to do with her weight.
Cato of Seekers is the hardest fighter of the group (male or female) and the most well endowed.
In Sequel SeriesThe Legend of Korra, the eponymous Avatar, who is by far the toughest female on the show also is very well endowed.
Lana Kane of Archer is the bustiest woman in the show and also the best fighter. Also, the other busty female besides Lana - Pam - turns out to actually be a fighter as well, rather than just an office worker.
Galatea of Justice League Unlimited is literally a clone of Supergirl, just aged slightly more. This means that she's a little stronger...and has grown about three cup sizes.