In Discworld the smaller races (such as the Nac Mac Feegle and the Dwarves) are explicitly stronger than humans because all the same amount of physical strength is focused into a smaller body.
The Gallivespians in His Dark Materials are a few inches high, but they have poisonous barbs on their heels that make them deadly in a fight. You do not want to mess with one.
Reepicheep and his relatives in The Chronicles Of Narnia, though they teamed up sometimes as well. A common tactic involved slashing various tendons and then, when the taller opponent fell, a quick slash across the throat.
Trumpkin and Nikibrik, the two dwarfs in Prince Caspian, were both skilled fighters, especially the former.
The Gruffs from Small Favor, the tenth The Dresden Files book, are troll-slaying goatmen from the fairy tales. After the five-foot tall first wave is defeated, their seven-foot tall older brothers take over - when they fail, the big brother that comes next is twenty feet tall - but the Eldest Gruff, most powerful of them all, whose coming shakes the earth and swats a fallen angel without effort? Five foot two, tops, with a limp. And if he didn't take a liking to our hero, Harry clearly wouldn't have stood a chance against him.
Not to mention Murphy (5 feet even, looks like a cheerleader, and took down an ogre with a chainsaw). The page quote is about her, with the subtext being that the Fists of God were debating recruiting her. She's done a pretty good job of establishing it as a potential second career and, hey, she is out of a job. Rebound career from cop to Knight of the Cross, amiright?
We also have the Red King, who is slightly over five feet and is quite possibly the single most Badass vampire in the world, with the only possible exceptions being the Kings of the other two courts at full power or rather, he was.
Compared to all of his enemies, Harry (and other Wizards) count as this. He regularly takes on things at the very least half his (admittedly greater-than-average) height again.
And then there's also Triel Baenre, daughter and successor of Old Matron Baenre, the Big Bad in many of the novels. While elves are not physically imposing to begin with and dark elves are even considerably shorter, Triel is frequently described as being exceptionally short even by the standards of other dark elves, which puts her, as effective queen of the most powerful nation of an Always Chaotic Evil race, at a hight of about 4'7, or 1,45m. In contrast, her younger sister Quenthel is always described as very tall and muscular, but never has attempted to gain a Klingon Promotion.
In the Warlock Of Gramarye series, the Wee Folk are to be feared, Puck especially. And dwarfish Brom O'Berin can outfight nearly any man in Gramarye.
Treecats from the Honorverse. Small, six-limbed aboreals, who look all cute and furry. However, they live on a world with a gravity of 1.6G, and have two definitions for enemies: those properly dealt with and those that are still alive. Add to that their reaction times and the fact that they have six limbs, each with surgically sharp claws, and there's a reason they've been compared to "berserking buzzsaws" when in battle rage.
A human being in good condition and with really good martial art skills had at least a fair chance against a treecat in a fight, simply because of the size disparity. But it wouldn't be easy and the human would certainly come out of it badly injured.
Lucky Starr's sidekick Bigman Jones is only five-two and one hundred and twenty pounds, but routinely wins fistfights against men who are foolish enough to underestimate him.
A minor character in the backstory of The Hobbit and The Lord of the Rings, Bandobras "Bullroarer" Took, was one of the tallest Hobbits in history at 4'5", and singlehandedly routed an army of Orcs by decapitating their leader with one swing of his club. (The head was sent rolling into a rabbit hole, thereby providing the origins for the game of golf.)
Humans, in the Prince Roger series, are a good 3 feet shorter than the 9-foot tall native sentients of Marduk. Their powerhouse-ness comes from their advanced tactics and technology.
Warrior Cats has Scourge, who is a very tiny cat often mistaken for an apprentice. He's also the leader of a group of stray cats known as BloodClan. Why? Because he can kill as easily as he breathes; his small size makes him extremely quick, and he's attached sharpened dog's teeth to his claws, making them that much more deadly.
Time Scout: Ann Vin Mulhaney, Sven Bailey, and Margo Smith are all petite and incredibly deadly.
Sisterhood series by Fern Michaels: Yoko Akia. She is 4 feet 8 inches (1.46304 metres) tall, she weighs 90 pounds (40.82 kilograms), and just about everything about her is tiny. She can flatten just about anybody in a fight and her fellow Sisters and Vigilantes call her "the 90-pound stick of dynamite"!
Played for the maximum possible amount of creepy in Alastair Reynolds' Century Rain with the Neotenous Infantry, better known as the War Babies. The Slashers, the spacefaring faction of humanity, have already engineered themselves for childlike bodies to make space travel easier, but the War Babies are childlike bodies specifically engineered to be lethal killing machines that ordinary human beings cannot bring themselves to shoot. The ones encountered in the novel are senescing due to (for them) extreme old age, so they look like children until you see their faces. They rack up quite a bodycount over the course of the story.
Adventure Hunters: Lisa is sixteen and strong enough to knock down stone walls. The older and bigger Artorius delegates all the heavy-lifting to her.
