Not a character, but in the same vein as the Defiant, the White Star-class of pocket cruisers from Babylon 5 packed advanced Vorlon and Minbari technology, along with 4 fighters. It was the first ship to be seen single-handedly and directly taking out a Shadow vessel, and it could also knock out capital ships much larger than itself. One of its initial advantages was that it could create its own jump points, something other ships its size could not do. Deploying multiple White Star-class ships into combat would often result in a Swarm of ships that would coordinate their firepower and speed to overwhelm any opponent.
Hawk is the shortest member of the Cobra Kai Power Trio, but also the physically strongest and most aggressive.
Bert is, not to put too fine a point on it, tiny. While he tends to lose fights due to his small stature, he has no physical or mental problems with keeping up with Cobra Kai's notoriously brutal training program.
Several of the Doctor's shorter incarnations, like Two, Seven and Thirteen, would qualify.
The 5'0" Zoe. She can judo the Karkus (three times her size) into submission and blow up an entire Cyberman fleet with just her brain.
The Sontarans are about 5' tall, but a Proud Warrior Race nonetheless. Don't bother trying to insult their height, though. They'll just bark, "Words are the weapons of womenfolk!" before killing you.
Ace is a teenaged delinquent, who might be only slightly taller than the above, but when the Daleks made the mistake of calling her "small?" Well, Dalek met baseball bat and Ace came out the better end.
"The Tsuranga Conundrum": The Pting is only about the size of a baby, but thanks to its invulnerability, poisonous skin, speed and appetite for all inorganic materials, it's a challenge to deal with. General Cicero mentions that she once encountered one that devastated an entire fleet.
Firefly. River Tam is easily capable of dropping many men much larger and apparently tougher than her, thanks to being a psychicSuper Soldier.
In "TOW The Girl Who Hits Joey", Joey's short, new girlfriend Katie playfully punches him over and over, which Joey admits really hurts. The others make fun of him for it until Katie does it to Rachel, who responds by kicking her in the shin.
Monica, who is the smallest of the gang but described as 'freakishly strong'.
Jon Snow, shorter than the sister to whom he is a fierce protector, but involved in all but one of the series' battle sequences, and is in the running as one of the best swordsmen of his time.
Howland is short, but also sneaky and tough and along with Ned is the only survivor of the battle at the Tower of Joy.
Hanna: Hanna is a fairly small teenager girl but highly trained in martial arts, and thus capable of taking down many opponents who outweigh her significantly. This also applies to most Utrax girls (none are that big) as they're teenagers as well who do the same thing.
Guerrero from Human Target. He only looks like a laid back computer geek; in fact he is a very intelligent, very dangerous ex-hitman. You do not want to get on his bad side.
The I-Land: Chase is a short and compact woman, but also a very skilled martial artist capable of taking down men who outweigh her by high margins.
Dutch is short and slim, yet highly trained in combat, beating up people far larger (including many men).
Calvert in Season 5 has much the same build, yet she takes on Dutch, Johnny and D'Avin at the same time, holding them off quite well for a time until she's overwhelmed. Justified in this case, as she's using stimulants of some sort.
Parker from Leverage. Having to do the things required of a professional thief, such as hanging from a ledge by her fingertips, has made her quite strong, despite being the smallest member of the team. When she thought Tara had betrayed the team Parker proceeded to grab Tara by the throat with one hand and hang her over the edge of a building.
Misfits: Seth's enforcer, a short, slim young man whom the gang initially scoffs at before he hurls them out effortlessly.
Justin Stewart in the Power Rangers Turbo episode "Little Strong Man", when he was bitten by a radioactive ant, but this prevented him from morphing into the Blue Ranger, so Justin had to give up his super-strength to continue being a Power Ranger.
Preacher (2016): Tulip, a petite woman with no trouble taking on opponents far over her size.
Djaq from Robin Hood. A popular icon of her image includes the tagline: "and then I ripped his lungs out..."
Dr John Watson who is a 5'7", mild-mannered, jumper-wearingkilling machine. Once punched the title character (6'0 and a badass fighter in his own right), splitting his cheek, then knocked him to the ground and put him in a choke-hold, all the while explaining that it wasn't a good idea for Sherlock to antagonise an Afghan war veteran who killed people when he had "bad days." He is also a very, very good marksman, since he killed Jefferson Hope, and, while tied to a chair at the time, killed the Chinese gangster. Oh, and threatened death on a professional killer when said professional killer grabs Sherlock in a headlock in an attempt to strangle him or snap his neck, which proves that, if you get this gentle soul pissed by threatening his friend's life-he WILL calmly, cooly kill you. The last episode of the second season has him breaking a guy's nose with a single punch, which results in a bloody nose. badass enough that it had to be hidden with a Gilligan Cut.
Series Three gives us Mary. Thought to be a normal girl-but-WHOOPS. Turns out she was an assassin from the CIA. Oh, and she shot Sherlock-which, mind, you, proves one thing: don't mess with her.
While the Prometheus, the first Earth starship in Stargate SG-1, is not very powerful and armed with (more or less) conventional weapons (regular/nuclear missiles and railguns), the second-generation Daedalus-class starships are considerably stronger, especially with the addition of the Asgard plasma beam weapons. The "pint-sized" part comes in once you compare the sizes of the Earth starships with the ships typically used by the various alien races. Even a typical Goa'uld Ha'tak is several times the size of the Prometheus (less so than the slightly-larger Daedalus). Then you bring in the enormous Ori motherships and the Wraith Hives. For reference, a Hive was once mistaken for a mountain. Unfortunately, a ZPM-enhanced super-Hive (even larger than a normal Hive) pulls a No-Sell on the Asgard weapons.
Taken Up to Eleven with the Lantean Puddle jumper/gateship. The ship is only a handful of meters long and seems like little more than a Starfleet shuttlecraft... but they are equipped with a cloaking device and armed with drone weapons that can bypass shields and easily destroy ships even as large as a Goa'uld mothership with just a few shots.
Not a character per se, but a ship: The U.S.S. Defiant, first of her class, from Star Trek: Deep Space Nine. Only five decks tall, 119 meters longnote For reference, the Galaxy class Enterprise-D is 640 meters and 42 decks, yet packing a massive wallop with experimental heavy weapons, powerplants suitable for ships several times her size, comparatively superior maneuverability compared to larger ships she can match in gunnery, and the ability to take as much she can dole out thanks to heavy armor. And as a bonus thanks to an intel-sharing deal with the Romulans, she sports a cloaking device. Ships like the Enterprise are meant to be all-purpose ships, and they're quite roomy and luxurious. The Defiant leaves out most of the science stuff and the recreational facilities, and the quarters are cramped pillboxes with bunk beds. She's a dyed-in-the-wool warship in all but name; officially, her class role is designated as "escort ship" in order for the Federation to remain politically correct. It's been repeatedly referred to as "a set of guns with engines strapped on." The Defiant is so overpowered relative to its frame that in the early tests of the prototype, the ship nearly tore itself apart.
Note, though, that she was never actually shown scoring an outright kill against anything bigger than a Breen cruiser; most of her kills were against ships her own size (although she tended to one-shot them). On the four occasions a Defiant-class went against much larger vessels, it crippled the target in the first case (Defiant vs. a Cardassian Keldon-class cruiser), fought the enemy to a draw in the second (Defiant vs. Lakota—though, notably, neither ship was trying for an outright kill; Lakota was trying to detain Defiant, and Defiant was fighting to disable her fellow Federation starship), forced the enemy to retreat in the third (Mirror!Defiant vs. Mirror!Worf's flagship), and was easily blown out of the sky in the fourth (Valiant vs. Jem'Hadar battleship).note It should be noted however that the Valiant was crewed by an inexperienced crew of cadets - with a seasoned crew it might have at least survived the engagement; if nothing else the fight was easily avoidable.
Basically, any warship in Star Trek that's not a ginormous juggernaut is this trope. The Klingon bird-of-prey, the original Romulan warbird from "Balance of Terror" (somewhat bigger than the Defiant, debuts the cloaking device and packs a Wave-Motion Gun), Jem'Hadar fighters, etc.
Species 8472 bio-ships probably take the cake. One such ship is considerably smaller than starships like Voyager, yet packs enough firepower to destroy a Borg cube with no more than two shots. Get nine of them together, and they can annihilate a planet.
Philippa Georgiou (played by Michelle Yeoh) in Star Trek: Discovery. Despite being only 5'3", Georgiou is a badass fighter, who can easily outbrawl much larger opponents. This is true to both the Prime and Mirror Universe Georgiou. The former was a highly decorated Starfleet Captain, while the latter has been the Empress of the Terran Empire, and is currently a Section 31 operative.
5'4" Michael Burnham (Sonequa Martin-Green) is also quite badass.
Supernatural brings us The Trickster, by far the shortest guy in the whole series, who not only tricks the brothers onto believing he's dead (more than once), becoming one of the few monsters of the week the Winchesters can't kill, but it's also revealed in the 5th season that he's really the Archangel Gabriel, one of the four most powerful beings in the show, posing as the pagan god Loki.
Russell Edgington from True Blood, a physically imposing man he ain't but stake his lover, and he'll go medieval on national T.V. He's 3000 years old (the oldest known vampire on the show), and vampires get Stronger with Age.
Wonder Woman: All of the Amazons of Paradise Island are slim, pin up girl physiqued women. All of them can physically overpower Nazi soldiers ("The Feminum Mystique"). Wonder Woman of course fights gorillas ("Wonder Woman vs. Gargantua"), alien killing machines ("Mind Stealers from Outer Space"), shape-shifting barbarians ("The Boy Who Knew Her Secret") and much more.
Sara Lance of the Series/Arrowverse is a petite woman who is also a highly skilled assassin. She is the smallest on the ever changing Legends roster and the deadliest, and at one point in the series even wields the raw power of death itself.