Power come from mass. Not from little man with muscles. If he not have mass, he not have power.
Force =
Mass x
Acceleration
You can generate a lot of force with small mass if its acceleration is high enough. Bullets are designed this way- small and light, but with high velocities...and some of the smallest rounds are among the most dangerous.
I see story. Chimp strong. But not punch. It scrape. It bite. It not punch for the boxing. Chimp is good for wrestle.
A monk's unarmed strike isn't just a punch, it is an abstraction of all kinds of various unarmed attacks a monk might make- punches, kicks, gouges, bites, rakes, joint locks, breaks, chokes, throws and so forth.
IOW, that halfling monk isn't just punching.
In addition, I've watched a lot of martial arts masters practicing their art. There was one featured in a documentary- kind of a "tour" of the world's martial arts. Of particular relevance to this thread, this small (5'1" max) old (early 70's) Vietnamese Master of some obscure martial art
may have weighed 130lbs, but generated about 45% more force with his strikes than an untrained but healthy man 1/3 his age and at least 45 pounds heavier.
The difference? 40 years of
training- learning how to strike, building up the density of his bones by doing repeated strikes to unyielding objects and the like.