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. 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."
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 two occasions a Defiant-class went against much larger vessels, it fought the enemy to a draw in the first case (Defiant vs. Lakota), and was easily blown out of the sky in the second (Valiant vs. Jem'Hadar battleship).
Basically, any warship in Star Trek that's not a ginormous juggernaut is this trope. The Klingon bird-of-prey, the Romulan ship from "Balance of Terror," Jem'Hadar fighters, etc.
Also 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.
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.
Rollo in The Cape, a midget who's also the strongman of the Carnival of Crime and capable of kicking the hero's ass in the pilot.
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.
Dr John Watson in the BBC's Sherlock, 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.
The last episode of the second season has him breaking a guy's nose with a single punch. Bad Ass enough that it had to be hidden with a Gilligan Cut.
Doctor Who has 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 in 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.
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.
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.
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.
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.
Friends : 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.
Also Monica who is the smallest of the gang, but described as 'freakishly strong'.
Cat on Victorious: Five feet nothing, harmless when you show hercandy, but get between her and her technology and she'll open a River Tam-worthy can of one-girl demolition derby on you.