Atom Ant. As well as Hal Seeger's Atom Ant expy, Fearless Fly.
The tiny donkey Zazoom from Arabian Knights isn't even up to the knee of the shortest knight, but if someone pulls his tail, he becomes a quite literal whirlwind of destruction.
Both Aang and Toph from Avatar: The Last Airbender.Especially Toph. She's the shortest of the Gaang, but she can levitate giant boulders into the air and smash them down on her enemies.
Bending aside, she's even shown to be the physically strongest teenager in the Gaang. This includes Sokka, whose shoulders her head barely reaches. She seems to be a match for Zuko, who is the oldest, and even taller than Sokka.
Sequel SeriesThe Legend of Korra: Korra displays this trait as well. When the White Lotus Society first meets her, she is already bending three elements with skill... at about age four. Even in the series proper, she is by far the shortest non-child character in the cast.
Additionally, Aang's three airbending grandchildren are capable of taking out large groups of trained adult opponents, despite the fact that none of them are older than 11 years old; the youngest, Meelo, even seems to be the best at combat of the three, despite being no older than 6.
Donald Duck. Many times, his strength isn't just due to his Unstoppable Rage, but also sheer determination. For example in "Canvas Back Duck", Donald manages to defeat boxing champ Peewee Pete by simply tapping him ever so slightly on his jaw (causing it to shatter like a window; Pete has a "glass jaw").
In "Tapped Out", similar to "Canvas Back Duck", but more deliberately, Donald manages to take down Pete as a pro-wrestling champ (when Mickey failed to do so) after the latter angered him after inadvertently ruining his nachos (which he does not like others to so much as touch).
In the Quack Pack episode, "Ducks By Nature", the camping leader is jealous of Donald and calls him "city wimp" but Donald carries both his own and the leader's camping equipment (which includes a canoe!) and then the camp leader challenges Donald to a race to climb up a cliff. Then, he cheats by secretly tying Donald to a tree with an extra-strong bungie cord. Donald struggles for a bit before his determination makes him so strong that he not only uproots said tree but literally SENDS IT INTO ORBIT and gets him to the cliff top faster than the camp leader!
Droopy Dog. Usually, his tactic is psyching his opponent out, or letting him beat himself up; but when push comes to shove, Droop will face down his foe; tell him, "You know what? That makes me mad", and proceed to beat the ever-living crap out of him.
In the short "Droopy's Double Trouble", the audience is introduced to Droopy's stronger twin brother, Drippy. The plot revolves around Spike running into Drippy (who has been instructed to keep out all strangers) and wondering why Droopy keeps beating him up for no reason.
Then in "Homesteader Droopy", his infant son ultimately thrashes the bad guy.
Garfield, normally not averse to eating live birds like he is to eating mice at all, is afraid to get near Ludlow, a small bird who is naïve and kind of annoying. However, he is not (known to be) this trope, or the reason Garfield's afraid. Ludlow's father, on the other hand, packs quite a punch, and while he's bigger than his son he's quite a bit more literally pint-sized.
In another episode, one mouse's mother is inadvertently trapped in a cookie jar. He won't believe Garfield hasn't done anything with her, but since Garfield doesn't know what did happen, he's powerless to prove otherwise. This proves dangerous, as the mouse brings over a murine parody of Arnold Schwarzenegger to beat a confession out of him. The Cup-Sized Powerhouse knocks him around a while before Garfield is able to discover and subsequently produce the missing mother mouse.
From the Justice League Unlimited episode "Dead Reckoning": Some of Grodd's legion of super villains under Lex Luthor's arrive at a Buddhist temple. They encounter a child monk, maybe ten or twelve, who tells them that they cannot enter. Lex sends the Atomic Skull, one of the largest henchmen he brought, to deal with him Skull tells the young monk he won't go easy on him just because he's a kid; the boy responds that he won't either, and proceeds to kick Skull's ass.
In Kidd Video, the gang's tiny fairy companion Glitter can perform godlike feats of strength, enough to lift entire buildings, when she "super sneezes", for a burst of a few minutes. Unfortunately, she genuinely has to sneeze in order to trigger it. Thus, whenever this ability is needed, she or one of her friends has to look for something like dandelions or a pepper shaker to make her sneeze, and there isn't always one within easy reach.
In the 1940s, Chuck Jones directed a handful of shorts featuring an African pygmy warrior named Inki. Also appearing in these cartoons was a small, black mynah bird who walked with a signature series of rhythmic hops and skips to the tune of Felix Mendelssohn's "Fingal's Cave" overture. The instant any attacker followed this bird around a corner (or otherwise out of the audience's line of sight), the sounds and music of a serious fight would ensue for approximately ten seconds, followed by the bird continuing on his way with nary a feather out of place. The attacker(s), on the other hand, were usually battered rather severely. To give one example, in the short "Caveman Inki", which was set in the stone age (described in the introduction card as being "probably before any of you people were EVEN BORN!"), the world's first last and only carnivorous brontosaurus was tied into several knots.
There was also Rocky the gangster, of "Rocky and Mugsy" and a few other Bugs Bunny shorts, who despite being a tiny man could easily beat up people much bigger than himself.
Kid Banty the boxing rooster from the Foghorn Leghorn cartoon "Sock A Doodle Doo".
The Mighty Angelo, the World's Strongest Flea, from "To Itch His Own".
Klokateer Number 216 from Metalocalypse is a very small member of Dethklok's bodyguards, but he is a very strong and capable fighter.
Mickey Mouse. Though not always victorious in his fights, Mickey has been known to jump into battle with much larger foes—Pete most often, but also the occasional monster spider (Gulliver Mickey, 1934), crooked Indian chief (Pioneer Days, 1930), or crew of pirates (Shanghaied, 1934).
Being small and living in a world of magic and running on the theme of Steven Ulysses Perhero, all of the ponies in Equestria are this by default, each of them having a special talent and power. Quite a few direct examples below:
Rarity. Unicorns aren't known for their strength, but she can drag carts full of gems with ease, and even carries a large boulder twice her size on her back. She also takes a martial-arts stance and then goes ninja on a bunch of changelings in "A Canterlot Wedding" (season-openers and enders tend to be more action-packed than the rest of the mostly slice-of-life series.) In fact, she's faster to resort to combat than Applejack (rough-and-tumble farm pony and physically the strongest of the team.)
Twilight Sparkle is another unicorn, and an early episode has Applejack and Rainbow Dash remark she's not exceptionally fit. Her magic blasts have the power of a tank gun.
Pound Cake is a Pegasus that can haul ponies twice his size while zooming through the air... and he's ONE MONTH OLD!
Maud Pie, Pinkie's Cool Big Sis, isn't any smaller than the rest of the cast, but she's certainly no more stronger-looking than your average mare. Turns out she can reduce boulders to dust with her bare hooves with ease.
"Crusaders of the Lost Mark" introduces a filly with enough Super Strength to lift up the schoolhouse with one leg.
Perry the Platypus. He's small, but packs a punch. At one point in the movie, he supports the combined weight of Phineas, Ferb, Candace, and Doofenshmirtz to prevent them from falling into lava.
Meap the alien . He's a tiny white snowman like alien with big blue eyes and a pink bunny ear hat, but is able to beat up opponents many times his own size and he is actually a police officer on his planet.
Also there's Agent S aka Sergei the Snail, who despite his small size and lack of limbs can somehow toss Doofenshmirtz around like it was nothing.
Scrappy-Doo does lift Scooby and Shaggy with one hand at least twice. So he has the strength for it... he just tends to borrow his strategies from Leeroy Jenkins.
Scooby-Doo, Where Are You!: The original Scooby-Doo's Velma was definitely this. Although she's the shortest member of the gang, she can carry all the rest of the gang and run from the monster at the same time.
Amethyst from Steven Universe is pretty strong even by Gem standards despite being as short as Steven. "Too Far" reveals that she should have been even stronger and Large and in Charge to boot because she's a Quartz Gem. However, she stayed in the ground too long and came out an "overcooked runt", as Jasper put it.
Nova from Super Robot Monkey Team Hyperforce Go!. She's a cute 2-foot-tall cyborg monkey who can instantly transform her hands into huge Power Fists that will knock any monster silly, as well as give her a monster grip. Back that up with what may be centuries worth of martial arts training, and you've got one Cute Bruiser who will never disappoint.
Starfire is tall, but quite scrawny (though really, everyone but Cyborg is, so maybe it's down to the art style), which makes it all the more hilarious when she gets dangerous and introduces villains to the full wrath of a starbolt-wielding Flying Brick.
In the Tom and Jerry cartoon "Jerry's Cousin", the titular cousin is called Muscles Mouse, and the cats where he lives live in fear of him.
Jerry himself becomes a Pint-Sized Powerhouse after a poison concocted by Tom backfires and gives him superstrength in "Dr. Jeckyl and Mr. Mouse".
And in "The Milky Waif", Jerry becomes this after Tom starts picking on his little nephew and triggers Jerry's Papa Wolf instincts.
In Transformers Generation 1 Brawn is one of the smallest Autobots (a little shorter than Bumblebee), but he's got muscle and attitude far beyond his size. Some of his exploits include picking up Soundwave and Megatron and tossing them like rag dolls.
Soundwave's Cassetticons (and later series' versions of those characters) can get pretty Badass for their size. The one everyone knows is Rumble: Human-sized, but able to shake the ground and bring giant-er bots down to Earth, as well as being able to hit as hard as any car or truck-bot. The one who takes the cake, is the Marvel comic's version of Buzzsaw, though. You probably don't remember the bird guy who was like Laserbeak but yellow if you've only seen the show, but in the comic, they were quite different characters. Laserbeak's job was a scout and spy, whereas Buzzsaw's job was to kick serious skidplate. The little guy had a habit of not just holding his own against, but curbstomping full-size bots. He considered himself an "artist" and his idea of a masterpiece was Autobots who'd been trashed with what he considered great precision and what everyone else considered "Holy Primus did he just take down OMEGA SUPREME?!"
The Transformers Prime incarnation of Laserbeak is the baddest yet. Definitely a case of Crouching Moron, Hidden Badass, he hardly seemed a character in his own right, deployed on extremely rare occasion to be Soundwave's eyes and ears and never being referred to by name. Then came Triage, when Soundwave left the dogfight he'd gotten into with Wheeljack's ship to Laserbeak. Minicon-sized bird-bot versus building-sized ship piloted by a Badass Wrecker; who would you bet on? In fact, Laserbeak brought down the Jackhammer alone and only took one hit. (The one hit, though, was enough to keep Laserbeak down long enough for Ratchet to hack him and get at some Decepticon secrets, but still, damn. Even so, by the end of the episode, Laserbeak was still mobile, and easily fixed. Not so the Jackhammer.)
Of course, don't think the Autobots can't pack a lot of mean into a little a space, too. One of Blaster's tape buddies was a rhino called Ramhorn, and he could tackle down any full-size 'con and boasted hip-mounted rocket-launchers (three missiles on each side). Now, if only he could do something about that awful stutter. Of course, nobody who's smart gives him any crap about it...