the bigger guy might have a more solid foundation, but he can still get punched in the head and if the lighter guy can generate the force required to shake up his brains, or maybe nail his liver, and because he's quicker can punch and get away, the extra weight means nothing.
As I said,
all things being equal, the bigger guy can punch harder than the little guy. Great punchers are born. Either you got it or you don't. It is all about speed and leverage. That said, F=MA still obtains. You can't change the laws of physics.
Sure the smaller guy can negate the big guy's weight advantage through boxing skills, which is why I agreed that Louis would beat this guy. That is a given. It is also a given that, except for the heavywieght class, we match fighters of about equal weight. Size does matter.