When I lived in Vegas several years back, I trained David Tua w/ weights before his fight w/ Danell Nicholson. (At that point in his career, Tua definitely should've been spending more time in other training modalities, but that's another story.) I was SHOCKED the first time I shook hands w/ Tua. His hands are TINY for a heavyweight fighter.
I asked his then-manager Kevin Barry how Tua could be the HW division's most feared KO puncher while having such small hands. Barry told me that having small hands actually made Tua the big puncher he is, because the force his punches generate is applied to a more limited surface area. In other words, it's all about the PSI (pounds per square inch) of the force.
I guess that makes sense, in a sense. On the other hand, some feared punchers (usually clubbers, like Foreman) have huge hands, so this theory obviously doesn't apply universally.
Thought experiment: assume equal force is generated by the punches of two fighters. One has fists that have a contact area of, say, 8.0 square inches, and the other has fists that have a contact area of 11.0 square inches. I'd guess the guy w/ smaller fists would be relatively more effective on point-of-chin shots, which cause KOs via a lever action that spins the brain within its cranium. But the fighter w/ bigger fists might well have an advantage on side-of-head and temple shots, since these punches are going to cause a whole lot of gray matter to bounce off the opposite side of his opponent's skull.
This is only my opinion, of course. I could be wrong.