Aliens: Vasquez, the smallest of the space marines, with the biggest gun.
Anna: Anna is a slender, petite young woman yet skilled enough at martial arts to kill multiple much larger men in hand-to-hand combat. She's in trouble when they get her pinned against something or under them, but always manages to break free somehow.
Mini-Me in Austin Powers. He's officially one-eighth Dr. Evil's size, and can take on Austin in a one-on-one fight. He's also a sneaky little guy.
Chucky of the Child's Play series is of course, a doll, which means he only reaches about knee high. But as several of his victims have found out, his small size doesn't mean he can't mess you up.
The Chronicles of Narnia: Reepicheep, the other Talking Mice, and the Dwarfs in the various Narnia films. (see Literature)
Negasonic Teenage Warhead in Deadpool is a small teenage girl whose powers basically make her a human bomb.
Sonja Stromberg from Doctor in Distress (1963) is short and plucky yet has a killer right hook as her boyfriend finds out after she dumps him.
Lori Campbell (Monica Keena) in Freddy vs. Jason. 5'1", and she decapitates Freddy Krueger with a fucking machete! But not before setting the dock on fire, blowing Freddy and Jason into the water.
Future World (2018): Ash looks like a fairly petite, slender woman, though due to really being an android with Super-Strength even using her bare hands she's capable of dispatching opponents far larger in the film without difficulty.
Mothra in Godzilla: King of the Monsters (2019), comparatively speaking. She is the smallest kaiju seen in terms of overall mass (with her wings taking most of it), but she's so powerful she can shatter the sky with her God Rays and there is a good reason she is called the Queen of Monsters as she is able to defeat Rodan and even have enough physical strength to slam the much larger and stronger kaiju into the ground.
Tinkerbell in Hook manages to take down several pirates effortlessly.
Clove, played by 5'3" Isabelle Fuhrman, toward the end manages to physically intimidate Katniss, played by 5'7" Jennifer Lawrence. Of course, right after, she gets beaten to death by the even bigger Thresh.
Peeta, played by 5'5" Josh Hutcherson, can lift a hundred pound bag of flour over his head and has no trouble lifting the much larger Cato off of Katniss and tossing him aside.
Hit Girl from Kick-Ass is 11-years-old and is able to mow through a small army of trained adult killers by herself, a pint-sized one-girl-army.
In Killer Klowns from Outer Space, the smallest klown knocks a grown man's head clean off with a single punch. Justified because the klowns are actually extraterrestrial monsters.
Sir Didymus in Labyrinth. He's a fox that rides a mid-sized dog and is an aggressive fighter, as well as a noble knight. The knight part means he's lacking a bit in the common sense department at times.
The titular character in Ant-Man. After being shrunk down, he retains his mass, meaning that the average strength of his punch focused on an incredibly small surface makes him act like a bullet. He has to learn how much strength he has to apply in a single punch so as not to kill everyone he attacks.
Natasha Romanoff aka Black Widow is tiny compared to the rest of the cast (Scarlet Johanson is 5'3"), but she still fights armies of burly dudes, kills multiple aliens (even wounding Corvus Glaive) and held her own against Super-Soldier Bucky Barnes twice. Her solo movie introduces another Black Widow, Yelena Belova, who is about the same height and an equally outstanding Badass Normal.
Bruce Banner, as his actor Mark Ruffalo stands at 5'7.5" which makes him the shortest of the male Avengers. This is fitting for a scientific Non-Action Guy like Banner, however pissing him off is a still suicidal decision as he becomes much larger, but Banner himself seems to retain some of Hulk's latent strength as he overpowers Scarlet Witch with one headlock in Avengers: Age of Ultron. Bruce also has Nigh-Invulnerability as well as he survives falling from helicopters, aircrafts, spaceships and crashing into Earth from space.
Rocket Raccoon in Guardians of the Galaxy, as long as he has a gun with him which also tends to be just as big if not bigger than him. (in the sequel he does go physical at one point, but more because he has the element of surprise)
Baby Groot in Guardians of the Galaxy Vol. 2 also fits the Trope; he is less than a foot tall, but still strong enough to knock a full-grown man down.
Valkyrie in Thor: Ragnarok is a rather small woman (her actress is 5'3.5", which makes her at least four inches shorter than everyone else in the main cast), but she's still an Asgardian with her species' Super-Strength and Super-Toughness, as well as a highly trained warrior.
Monos: The Messenger is a little person, no taller than 4'8, yet he's absolutely shredded. Whenever he visits the Monos, he drills them strenuously on their physical training. The actor who plays him is a real life example. In spite of his size, he was an elite soldier in the Revolutionary Armed Forces of Colombia for many years.
Moonraker: After Jaws' cable car crashes, he gets pinned under a huge gear that even a hulking brute like him was struggling to lift. Then Dolly, a tiny girl, helps him and they lift the gear off him.
Morgan: Morgan looks like a petite girl in her late teens, but is capable of taking down a far larger man with ease, because she's an artificial human with Super-Strength.
Prey: Zigzagged. Naru relies more on the tools at her disposal and exploiting weaknesses to gain a footing over her enemies. Near the end however, she's able to pull the gargantuan Predator into a pit of quicksand despite having a short, slim build and being dwarfed by it heavily. At the very least, she wraps the narrow rope around its neck, well above its center of gravity.
In Real Steel, Atom is an old robot a few generations behind the current fighters. He is also considerably smaller than them. With some modifications by Max and boxing training by Charlie, Atom is able to own most of the competition.
Just about any character played by Yayan Ruhian, a 5'2" Indonesian actor and martial artist. He tends to play villainous roles, in an unsettling inversion of Evil Is Bigger. The Raid in particular emphasizes his Psycho for Hire character Mad Dog as the best fighter in the movie, despite his opponents towering over him.
Space Jam: Nerdluck Nawt may be the smallest member of the aliens, but that does not mean he can pack a punch.
Star Trek (2009): L'il Spock. He beats up a bully twice his size for insulting his mother and scares the other two bullies from helping their friend.
Yoda: Size matters not. Look at me! Judge me by my size, do you? Hmm? Hmm. And well, you should not. For my ally is the Force, and a powerful ally it is.
Ewoks. Cute little teddy bear-like aliens who can overpower combat trained humans in hand-to-hand combat with little effort. During the Battle Of Endor, some Ewoks can be seen throwing Stormtroopers around like ragdolls!
Princess Leia in the original trilogy and her mother Padmé in the prequel trilogy are played by 5'1" Carrie Fisher and 5'3" Natalie Portman, respectively, but they are both competent Action Girls who can hold a fight. Shortness and ass-kicking combined seems to run in this family: according to The New Essential Guide to Characters, Luke Skywalker is 5'6". Outside the Skywalker family, Rey from The Force Awakens is the same height as Luke, but that's rather average for a woman. Even so, Rey is quite small and slender, yet handy with a stick (also later a lightsaber).
Emperor Palpatine by the time of Return of the Jedi is also 5'6", down from an original height of only 5'8". And he's still the most powerful, terrifying Sith Lord of all time.
Rogue One also features a vehicular version in the form of the Hammerhead corvette, a Rebel ship a fraction of the size of a Star Destroyer but with an armored bow and engines powerful enough to ram a disabled destroyer through a second one and into the shield gate, destroying all three Imperial ships.
Jason White from Superman Returns. He starts off as a wheezing asthmatic child, child of Lois Lane and her new squeeze. Then crushes a mook with a thrown piano when said mook threatens his mother, proving that his real father is Superman.
Sweet Girl: Rachel is a short, fairly small young woman who remains capable of holding her own against bigger opponents due to the intensive training in combat she received from her father.
Parodied in The Three Stooges short Punchy Cowpunchers. When each of the mooks tries to take away the heroine Nell (played by Christine McIntyre, who was about 5'2" in real life), he gets knocked out with a single punch. And in true ladylike fashion, Nell faints after each struggle.
Turbo: A Power Rangers Movie: After he accidentally learns the rangers’ secret identities, pre-teen Justin is recruited by Zordon to fill the blue ranger slot left vacant by the currently-incapacitated Rocky. When he morphs, his body is shown as increasing in height.
Luther, played by 5'6" David Patrick Kelly in The Warriors. While his outward personality is that of an obnoxious pest, he's actually a very dangerous individual, as you can work out from the "Oh, Crap!" expressions on the Warriors faces when he shows up at their hideout.
beat up a couple of bikers.