The extremely shy Ururu Tsumugiya wields a bazooka that resembles a giant lit cigarette as a weapon. The weapon is bigger than she is.
Soifon's Bankai is a giant missile launcher that's attached to her arm. It's so big she can barely move when carrying it, and needs to tie herself down with a heavy iron sash to stop the recoil blowing her away.
Fledgling Vampire Seras Victoria from Hellsing is frequently shown waving around her 30mm artillery rifle which is longer than she is tall. Later in the manga, she starts shooting down zeppelins using a pair of 72mm automatic versions of those rifles. Yes, Small Girl, More Dakka! And Akimbo! Near the end of the manga, she picks up an 88mm artillery cannon and fires it off the shoulder into the Major's face. To a lesser extent, Rip van Winkle, who wields a very large (albeit not excessively so) musket.
Rally Vincent from Gunsmith Cats usually goes for quality/precision over raw power/rate-of-fire, but she will pull out the occasional BFG when needed.
Canon material says that she is 129 cm (roughly 4'4") tall and weights 24 kg (roughly 52 pounds). In the movie she levels a city with a single Starlight Breaker. That tiny girl isn't called White Devil for nothing.
Dieci and Wendi have large guns as part of their Inherent Equipment, the former being a BFG that can fire devastating explosive projectiles, and the latter being a surfboard-like gun that she can use to fly through the air, fire energy bolts and block attacks. In one part of the manga, Nanoha is shown testing out a gun that appears to be around her size.
Arnage wields multiple Gatlings and missile launchers.
The Gretel personality from Black Lagoon can fire a Browning Automatic Rifle (with a teddy bear or doll keychain attached to the barrel) from the hip despite only being about twelve years old. There is some real-life precedent for this. Bonnie Parker (of Bonnie and Clyde infamy) is reputed as having fired a BAR from the hip, and she was quite small at 4'11" and 90 pounds. The BAR was noted for having fairly low recoil, as it was heavy (around 20 pounds loaded) and had a recoil-reducing mechanism in the buttstock.
Gunslinger Girl; unsurprisingly, given the name. Although any of the glamour or comedy typical of the image is subverted about as hard as it possible to get. The first DVD volume is even titled "Ragazzine Piccole, Armi Grandi" — literally "small girls, big guns" in Italian.
Palus Abel from King of Braves GaoGaiGar has multiple large energy cannons attached to her chest that conveniently only appear when she spreads her cloak.
Mana from Mahou Sensei Negima!. Half subverted: the guns are air-powered replicas (most of them, as the biggest is confirmed as an actual anti-materiel rifle), but they use ridiculously effective magical ammunition. At the end of the Kyoto Arc, Chachamaru uses a gun that's at least as long as she is tall.
Comes up from time to time in Eat-Man given the way the setting is saturated with BFGs
In Seto no Hanayome, Maki's Water Cannon is only the size of a conch-shell but the proportions work out.
In the second season of Code Geass: V.V is a Small Boy with a Big Gun, which he uses to murder his sister-in-law. Also, the gun is not that big, but V.V is so tiny and it fires so rapidly that it looks quite bigger than it really is. Anya is a sixteen-year-old girl and the youngest person to ever be accepted into the Knights of the Round (the Britannian Empire's twelve most elite soldiers) and whereas most Knights of the Round pilot swift, agile custom mechas, Anya's Knightmare Frame is really slow but has impenetrable shielding and countless guns, including one BFG that probably makes up a third of the frame's weight. Toward the end of the series, Nunnally gains control of the Damocles, which is basically a mile-long gun that fire nukes.
The twins Luki and Noki from Dogs: Bullets & Carnage are about twelve years old and ridiculously cute. They also have cybernetic arms that pack massive guns and blades that can and will reduce you to a smear of red paste.
Defied in Desert Punk. Kosuna starts bugging Kanta and their weapon shop owner to give her a bigger gun. After the owner has her dig a lot with an oversize shovel, she tries out a moderately-sized rifle. Even though she's a pretty good shot with it, she realizes it weighs too much for her to carry it and extra ammo around easily and the recoil is so strong it actually hurts her shoulder. Her and the owner then conclude that you need to know your own limitations and guns are just tools which become pointless if you are unable to use them, so she settles on a more powerful but appropriately-sized 9mm automatic weapon (which is made a point that if she's doing her job right she wouldn't even need since she's suppose to be staying out of sight).
Eboshi in Princess Mononoke believes that women should not be excluded from warfare and has her gunsmiths design a rifle for women. It is fired more like a rocket launcher than a musket and her personal prototype seems to be the most powerful weapon in the whole film.
Himeko of Kurohime wields the legendary pistol Senryuu (Tornado Dragon), a huge pistol capable of firing magical bullets. Additionally, it shoots regular bullets with enough power to wreck a small carriage. Ten years earlier, she wielded a ridiculously long rifle that was capable of summoning dragons without any ambient magic power.
Kaori in City Hunter, except that she carries a ton of them yet even with this great arsenal can't even hit a single baddie. This can be extended to Xiang Ying in AngelHeart as she inherits Kaori's Heart and the first thing she does is to blow up a castle. She can later be seen with all sorts of automatic weapons as she is a former assassin.
One scene in episode 5 of FLCL has Amarao's female sidekick Kitsurubami trying to take out Canti the robot with an anti-tank rifle.
Homura Akemi's not to be forgotten. A 14 years old girl who fires machine guns and rocket launchers with ease. Stolen from army bases thanks to her time-stopping power. By the end of the series, she stole and fired cruise missiles. (Granted, she wasn't holding them in her hands, but still.)
When a semi-paranoid Spider tells Channon and Yelena to get guns to defend themselves, streetwise Channon takes them to a gun dealer, and patiently explains to a somewhat condescending proprietor, in technical terms, exactly what is needed. Contrasted with her is the diminutive Yelena, who finds a bazooka-like gun and asks if she can have it. (The proprietor declines, and insists it's not because she's a girl, but because it was designed for people with two backup spines.)
Stargirl of the Justice Society of America and the Cosmic Rod, which is about a head taller than she is (since it was made for Jack Knight) and a very potent weapon and sky surfboard.
Tank Girl, in both comics and film, has a love affair with this trope. To emphasize the phallic, she's often shown standing waist up from the tank's hatch, with the barrel of her Big Gun aimed forth from where her pelvis would be.
Brianna Diggers from Gold Digger might not be small, but she uses guns and Power Armor that are far larger than her with ease and, later on, tends to power them with magic.
Sabretooth's partner Birdy, from Marvel Comics
Little Victory: An early 'test' for Victory Girl was Boomstick. She might not be smaller than normal, and what she wields is not a traditional 'gun', but the huge symbiote cannon that is her right arm does seem to count.
Panda Delgado in Body Bags. Mostly only in covers and pin-ups, in the actual book she usually has a Glock.
As part of her superpowers Jules from Bazooka Jules can summon any weapon or gadget she wants. There doesn't seem to be a limit as some of the guns she summons are bigger and wider than she is. Thanks her enhanced strength, she can hold them with ease
Private Vasquez from Aliens is played by a 5'3 actress and wields a 40-pound machine gun. "Let's rock!!"
Little Rock from Zombieland can use a quite impressive arsenal for a 12-year-old.
In Predators, one of the character is a Special Forces sniper played by Alice Braga. The actress went on the record as saying that she had to train a lot just to be able to carry the gun around.
Nikita carries a Desert Eagle. Lampshaded in radio adverts for the film, where Antoine de Caunes said, more or less, "If you like movies about pretty girls in teeny tiny dresses carrying big guns..."
Ma-Ma in Dredd mostly plays with blades, but when she uses a gun it's a really big one.
Hope Adams from Women Of The Otherworld has the superpower of sensing chaos, but no combat abilities. She's also quite shor- petite. Since her chaos detector invariably draws her into danger, she makes a point of carrying at least one gun with her, and she regularly practices with it.
Shiro Miaowara of The Adventures of Samurai Cat is male, but fits the trope otherwise; he's a tiny little boy, and packs ridiculously huge guns. At one point he totes around a giant vulcanAvenger cannon and says he's able to swing it about thanks to possessing the strength of madness.
Zack of the Hell's Bells series of Urban Fantasy novels is an example of Small Girl Big Staff, her channelling focus being an iron pole described as longer than she is tall, with which she wipes the floor with vampires, wizards and demons alike. As if that weren't enough, her normal firearm is a .455 Webleyrevolver, and in the climax of Dead Silver she ends up behind the barrel of a minigun.
Notably averted with Lt. Karrin Murphy of the Dresden Files. She's a 5-foot, petite, blond, where the shit did she pull a gun and a badge from looker cop. She carries appropriately fun-sized guns. Dresden ribs her for it repeatedly. Next to her? Human linebacker-mountain Hendricks, as well as supernaturals of all sorts, who enjoy packing absurdly-high-calibre guns. Dresden describes them as anything from "assault rifle" to "ICBM". One of her guns (a highly illegal P90) fits in a box of chocolates. That she got from Kincaid. To remember Hawaii.
Played straight with every female White Court Vampire.
In the Belisarius Series, Antonina's Hand Cannon is large enough in relation to her that every time she uses it, the recoil is described as spinning her around or knocking her on her ass.
Live Action TV
Any number of police/thriller shows where a petite female character wearing body-hugging clothes suddenly pulls a full-frame service automatic from under her jacket. Caitlin Todd (Sasha Alexander) often did this in the first two seasons of NCIS, and Rita Lance (Mitzi Kapture) of Silk Stalkings did it in nearly every episode. Avoided in the serial-killer thriller Copycat where very petite Holly Hunter carries a compact SIG-Sauer 232 in a realistic manner.
She isn't exactly timid but Ace from Doctor Who could be an example of this trope.
And the S4 episode The Stolen Earth has Rose sporting a rather big anti-Dalek gun.
Rose has a massive ice gun (well, a futuristic fire extinguisher) in "The Girl in the Fireplace"
Gwen Cooper from Torchwood Miracle Day wields one magnificently.
In an episode of Everybody Hates Chris, Chris has an Imagine Spot of his family manically holding weapons, with each descending family member getting a bigger weapon, leading up to Tanya holding a bazooka.
When testing if you could bullet-proof a vehicle with phone books, Kari got to play with a .50 cal sniper rifle, and succeeded in disabling the engine of the vehicle with it.
Science gets more fun when I get a bigger gun.
Also in the James Bond Propane Tank myth, where the boys had a handgun and a small-caliber rifle, she brought in a shotgun!
In fact, just about any time all the members of the build team are shooting something, she's the one with the biggest gun. This probably isn't accidental.
In the "Scary-go-round" myth, Kari fires a .50cal rifle, as well as posing with it.
Dr. Brennan of Bones carries what appears to be a stainless .45 revolver for a few episodes, prompting nervous commentary from Agent Booth. In a moment of unusual realism, it get subverted in a later episode; in the midst of a desperate firefight Bones offers Booth her gun, admitting "mine is too big for me." It's a Smith&Wesson .500, the largest caliber handgun available on the market. It's used for metal silhouette target shooting. And moose hunting. And shooting bad guys through heavy steel doors.
Despite being a size 0, Fiona from Burn Notice always has a huge shotgun or assault rifle handy nearby.
It's not her weapon of choice, but Buffy once takes out a particularly powerful demon with a rocket launcher almost half as big as her body.
Kamen Rider Den-O: Hana may be a young girl due to timetravel weirdness, but her weapon of choice is a bazooka. No weapon is more fitting for a girl who can singlehandedly beat the tar out of a kaijin twice her size.
Although not always small, and rarely qualifying as "girl", the few times the folks in True Blood wield guns they are hardly proportionate; somewhat justified in that the wielders are either superhuman (vampires, werewolves, etc.) or are FIGHTING the superhuman. Or both. Pam takes the cake when she tries (unsuccessfully) to take out the witches' protective shield-spell with a rocket launcher.
"Gunpowder & Lead" by Miranda Lambert – played literally. The song is about a tiny woman who suffers horrendous physical abuse by her much larger, stronger boyfriend. The boyfriend is sent to prison as a result and now has been released. The girl learns that he is out and is coming home ... but she plans to be waiting with a surprise of her own and to show her boyfriend (in a deadly way) "what this little girl's made of."
The character of "Suzie Jones" in Vampire: The Masquerade. Embraced at the age of 16, Suzie was your archetypal gun-bunny munchkin Brujah. Used the Obfuscate discipline to wear all her firearms in plain sight, and just keep people from noticing. Highlights of her career involved firing an anti-tank missile launcher down a crowded tunnel (it was full of demons), using her fangs to literally hang off the shoulder of a much larger vampire elder in melee while using her sole functioning arm to stuff an unpinned white phosphorous grenade into the elder's back pocket, and going through an entire 500-round belt of minigun ammo while in a synagogue. (In her defense, she was aiming at a Setite Methuselah that had already flattened the rest of the party, and her only hope of survival was More Dakka.)
Like all things Warhammer 40,000 the sisters of battle take this up to eleven by being an army of normal women in "7-foot-tall-Space Marine" grade power armor with a preference for "7-foot-tall-Space Marine" weapons. Particularly the ones that Melt and Burn things.
Taz from Starship arguably has the biggest gun in the group despite being the shortest person in the cast.
Any female character from Gods Eater Burst fits this description, as the weapon they all wield, the God Arc, is several times their size by standard.
B. B. Hood from Darkstalkers, who carries machine guns, land mines, flame throwers, grenades, a rocket launcher and occasionally, a pair of heavily armed mercenaries under her red riding hood cloak.
Rozalin from Disgaea 2: Cursed Memories. While, technically speaking, she can be equipped with anything from a sword to a staff like anyone else, she unarguably is best with ballistics and her first special attack involves whipping a chaingun bigger than she is out of Hammerspace and riddling enemies with tons of bullets.
In Makai Kingdom, just about anyone can wield a minigun, bazooka, or giant arm cannon though only a few classes are proficient. With class changes, an Archer can use those just fine. Etna specializes in miniguns as a special character, while Castile can use bazookas.
Try handing Miranda or one of the female Marines one of the bigger weapons in Halo. You'll suddenly realize that they were initially scaled for 7 foot cyborgs or 8 foot aliens.
Take a wild guess how a gender-bender version of Heavy Weapons Guy is usually portrayed in fanart. And no, they didn't make the Scout a lumbering beast in return.
Carol from Wild ARMs 5. Teeny-tiny girl, huge honking rocket launcher.
Maya Schrodinger from Wild ARMs 3, not quite tiny but certainly a petit, has a hidden minigun as part of her arsenal. Hidden...well...see for yourself. (Skip to 5:33)
Xenosaga has a couple. KOS-MOS isn't exactly small, but she dual-wields triple miniguns in a few attacks; Shion plays it straighter, with her arm-mounted MWS.
Lady from Devil May Cry hefts around a gun that happens to be larger than she is tall.
Any female Lancer from Valkyria Chronicles, whose weapons are longer than they are tall. Applies to Aisha as well, as she's only 12-years old and wields the Shocktroopers' standard assault rifle.
Carmelita Fox in Sly Cooper wields a shock pistol that kicks so hard she can hardly hold onto the thing when she fires it. The bolts of electricity it fires can destroy inanimate objects in one shot and kill even hulking flashlight guards in 2 shots.
Iji complains at the beginning that she can barely lift her Nanogun. Hardly surprising, as it's designed to be carried by a Tasen Soldier, who are about seven feet tall. Iji herself is actually 6'3", but the weapon is still pretty big compared to her. The recoil from the MPFB Devastator (essentially a triple-nuclear-rocket launcher) knocks her off her feet, unless she maxes out her Strength stat.
Possible via cheating in Fallout 3. Use mods or console commands to make your character a 10 year old, then specialize in Big Guns. Normal adult female characters specialized in Big Guns also fall under this trope.
Femme Fatale Mona Sax from the Max Payne games is rather slender and petite, yet her signature weapon is a Desert Eagle, the largest handgun available in the games. This is especially evident in her Mexican Standoffs against Max, who usually sticks to more modest but more efficient (dual) Berettas.
Tess designs BFGs for Jak that are at least as big as she is. And if she continues her gun-designing career after her transformation into an ottsel...
The unlockable cutscene commentary in Jak 3 explains that, in one scene, they actually had to scale down one of Jak's guns just so that it wouldn't look ridiculous when she held it. Considering how many fans this trope has, it's kind of a shame they didn't leave the gun as it appears in-game.
Playing a female Shepard in Mass Effect 2 and using the Cain probably counts. The weapon itself isn't THAT big, but it makes a ridiculously big explosion. Likewise, any combat-class FemShep who picks one of the advanced weapons. The Widow looks like a telegraph pole, and the Claymore and Revenant are just huge.
Also, when she's first seen, she takes a gun from Snow to defend the group of Purgees with. Bonus points for childishly aiming it at Snow and saying "Bang! Bang!" in a six year old's voice.
There is also a cutscene in Chapter Nine where she steals a soldier's rifle and unleashes her fury on a Vespid Soldier in order to escape with Sazh.
During the first chapter, Snow recruits Hope's mother Nora to fight against the soldiers. She ends her brief appearance using a rocket launcher to take down an airship, citing "Moms are tough". What happens next though...
The first chapter also features Lebreau as a Guest Star Party Member. While Gadot (another guest) carries a normal military rifle, hers is much larger.
Playing Rochelle and picking up an M60 in Left 4 Dead 2 could count as this, seeing as the M60 is almost at tall as she is.
A build possibility in Hellgate: London; a tiny markswoman can dual-wield massive gatling rocket-launchers.
Phantasy Star Online has the human and CAST female Ranger classes, R Amarl and R Acaseal respectively. Phantasy Star Universe also has a Ranger class and its offshoots that are open to females of any race. While not always being the case, they can be made as small as a young girl. And with the same customization systems, one could also make a Gender Flipped example of this trope.
Same in Phantasy Star Zero - and the cannons of that game compensate for every penis on the server.
Rebecca Chambers from Resident Evil 0 becomes this when equipped with a shotgun. It's not that the gun is big as much as she's really, really tiny, but the result is still pretty hilarious.
Jill Valentine in Resident Evil 3: Nemesis isn't as tiny, but her bonus Rocket Launcher is only slightly smaller than her.
Aya, the small-framed heroine of the Parasite Eve series gets her hands on some really big guns. In the first game she gets a rocket launcher that is as long as she is tall. The sequel ups the ante by allowing her to wield a giant portable railcannon that takes time to charge, and sends her back a few feet whenever she fires it.
TimeSplitters 2 lets you dual wield a fair number of weapons. Picking any of the smaller characters and using twin homing RPGs can lead to a few laughs in multiplayer. Unfortunately, the full potential is prevented by scaling.
Saki Omokane from Quiz Nanairo Dreams.
In Intrusion 2, the boss Maku is an average size but very thin woman who wields a positively massive laser cannon.
Fio is the smallest of the four main playable characters in the Metal Slug series, but not only is just as capable of rocking every uber-powered gun in the game as the other characters, but in later games becomes the heavy weapons specialist who starts every life with a Heavy Machine Gun.
Ciel from Tsukihime, a curry loving Meganekkoby day◊ and... this◊ by night. Plus she's a bit of a Knife Nut. Though her "Seventh Holy Scripture" is not so much a gun as it is a pile-bunker with a unicorn's horn for anti-vampire purposes... but it's close enough. Normally, the magical weapon is a spear or sword, but she personally and specifically redesigned it to be an enormous jack-hammer. She's just that hardcore.
Homestuck: Jade Harley is an adorable thirteen-year-old Genki Girl... who happens to very adept at the use of hunting rifles as long as she is tall. When she gains access to Item Crafting, the guns get even bigger.
Panda Xpresshad its origins in a sketch of the (five year old) protagonist Dahlia with a machine gun. The gun was replaced with a cyborg panda before the launch of the series.
Bunker, of the Whateley Universe. Petite (mutant) girl, usually armed with bazookas, full-sized machines guns, you name it. She likes the big guns. Hive might qualify too, given that she wields a Barrett .50 caliber sniper rifle.
Ruby Rose of RWBY carries a scythe/sniper rifle combo that's one-and-a-half times longer than she is tall.
Disney seems to have a fixation for girls/love interests with guns who love to use them:
Glittering Goldie from DuckTales. In Scrooge's nephews' words, "Cupid's arrow is nothing next to Goldie's shotgun."
Most of the main female characters in Danny Phantom has brandished large Fenton guns at several points in the episode. Especially Ghost Hunter Valerie who had rocket launchers attached to her shoulders at one point!