@twinklecake A lot of what you say is true and I agree but it is not as simple as that. If two people have similar skill sets then the heavier one would almost certainly win. But when the skill sets are very different, then there are a lot of other variables that comes into play, although weight is still a definite advantage.
For example, in MMA when the Gracies first made their appearance, they were dominating much bigger guys who are talented but lack experience in their type of grappling. Even among two strikers, size is not everything as you can see in the Cro-Cop vs. Bob Sapp match, or even in boxing when Roy Jones stepped up to heavyweight and beated John Ruiz for his belt.
I believe Mike Tyson would have a tremendous advantage over Bruce Lee as well, but who knows if a kick to the knees to someone who never learn to defend it would do to them. Or like a great boxer like James Toney who was like a fish out of water when Randy Couture took him to the mats in an MMA fight.
If a smaller guy knows how to throw a proper punch, it is unlikely he would hurt his hand punching a bigger guy in the chest. Most people would hurt their wrist or jam their fingers from throwing an improper punch instead.