When a small person is accosted by a larger opponent, most people expect the latter to win, but in fiction, the smaller a combatant, the more ass they can kick.
Like the Square-Cube Law going down, a combatant gets cooler and more badass the smaller they are. Short guys trounce giant bruisers, children routinely trounce goons, little people will be amazing wrestlers (if not literal ankle biters), and mice, rats or other small creatures (especially of the Talking Animal variety) will swarm and overwhelm, if not individually disable enemies with rapier precision.
Much like Muscles Are Meaningless and Crouching Moron, Hidden Badass, these characters are formidable fighters despite (or sometimes because of) their size.
There is some basis in science for larger proportional strength for smaller creatures: the Square-Cube Law works in the other direction. Although the volume, mass, and inertia of a half-sized humanoid such as a Hobbit is one-eighth a standard-sized human's, the strength of muscles is based on their cross sectional area, making Hobbits proportionally twice as strong as a human, and much more agile. This is also why ants are so strong compared to their mass. However, for this trope, Rule of Cool is in play whenever the Pint-Sized Powerhouse is as strong as, or stronger than, an average-sized human.
See also Apron Matron, for when small old ladies do this, and Mini Mook. Cute Bruiser, for when they're young, or at least very young-looking. It's also a common trait of the Mentor in Sour Armor. Killer Rabbit covers "harmless" animals. See Weak, but Skilled for a common explanation behind this. Waif-Fu and Glacier Waif are subtropes. This Trope is often - but not always - associated with a David Versus Goliath fight.
