I think it's much more than big! you better know how to fight, have the right style to beat your opponents style, be smarter so you know what ur opponent will do before he even does. Be naturally fast and naturally hit hard, then get trained to hit harder and move even faster. In short size is just 1 factor out of many, anyone smart, who's actually been in fights know all this! I've seen dudes 220lbs cut that can't hit hard enough to hurt a light weight! I don't know why this is, but it is. Just like some lighter dudes can lift heaver than their body size would indicate, meaning some small guys are stronger that guys 2x their size!
In short
1.size is a factor but doesn't indicate strength.
2.size doesn't indicate how hard you can hit.
3.size dose not indicate skill.
4.size does not make you faster.
5.size does not give u better cardio.
6.size does not make u smarter. (allowing u to play chess instead of checkers)