I think the legs (and how you use them) is often more important. (By 'body' I'm assuming you were only talking about the upper body.) Look at Hamed back in the day -- didn't look particularly muscled at all, in fact he just looked like a small, skinny guy, and yet many people regard him as being one of the hardest punchers ever to grace the featherweight division. Part of it was due to his powerful legs and the way he would leap into shots or really twist with his legs and use unorthodox motion to generate power into things like uppercuts. For a more modern example, look at Pac's legs: http://www.eastsideboxing.com/forum/showthread.php?t=183907
think as mentioned the legs are most important as they dictate what the trunk does, on hearns he had a body built for speed which equals power
While I pointed out how important the legs are, I didn't say they were the ONLY element -- there are all sorts of things in play when it comes to punching. We can guess at a few of them, but there are always more mysterious factors and intangibles. I think the only way we can do it is to look at it on a case by case basis, as every human is obviously unique and different. In regard to Hearns, he was blessed with excellent speed and long, whip-like arms. That in itself doesn't equal power, but I think his punching form was really good -- when he throws his power shot he pivots and completely twists his shoulders and back into the punch. He puts his entire body into it...you can actually see him trying to teach Bradley how to do this here: [ame]http://www.youtube.com/watch?v=g1GtrWw09eE[/ame] Also notice what Bradley says after the session with Hearns: 'He was telling me (...) I gotta turn my punches over, shift into my punches...that's where the knockouts come, from turning all the way into the shot...I'm gonna take this advice and definitely try to use it in the fight.'
When it comes to punching power its all in the hips. Its all in the hips. What? You guys thought Weathers was just talking golf? Well, not really in the hips, but the waist. Seriously, you look at the biggest knockouts it all has to do with gyration of the hips, you can be ortho, square, southpaw, doesnt matter. As for tough boxer pillow punches no fighter punches like a pillow, but some harder than others. Featherfisted Malignaggi and Jimmy Young come to mind.
Wasn't only speed.. He was a strong and powerful guy period. His right hand at 147 was lethal! Some guys just got power and I have no idea where it comes from. Hearns was muscular but Trinidad was not (not the same way at least) and he was an incredible puncher. Malignaggi is always ripped and muscular but no power.
Alexander appears too "tight" to let his punches flow correctly to get power on them. He looked like his whole body was cramped. Long lean guys generally have some pretty good pop on their punches. Too many variables to really know. I guess that's why they say, punchers are born that way.
Muscle mass is secondary in boxing, power comes from proper technique and delivery with means you have your whole body, weak looking as it maybe, set in proper motion and concetrate all the energy to the very moment and place upon connect. For the best punchers everything clicks, the timing and placement are perfect, power comes from all parts properly set by footwork and the connect is at the peak of the motion, not too soon, not too late. For some it all comes naturally take Tyson, Tito or Morales let's say. Others need to learn to punch. Of course some compensate the lack of perfection with muscle mass like Abraham let's say, but they still need to put the motion pretty well together. Muscles mean little, you are the weapon. :yep