Sources of their power...

some skinny fighters just had strong fast twitch muscle fibers for power others used there strong legs to help launch there bodyweight wile others had specific muscle groups that were huge like tysons uppershoulders andchest and back dempseys monsterous deltoids and tommy morrison had huge shoulders,traps deltoids ect. ect. it just depends on the fighter and there punching style and the angles of the punches

 

Prince Naseem Hamed was the hardest hitting featherweight I've ever seen, and if you look at any of his fights, it is apparent how much thicker and more muscled his legs are than any of his opponents. The Prince wa short and didn't have great technique, but the anchor of this extra muscle allowed him to fire off knockout shots from a standing position.

 

Tommy Hearns was wiry strong with tight muscles as a fighter. Power is often attributed to lower length strength but a lot of it has to do with proper technique IMO.

Want to learn? Watch Alexis Arguello throw right hands in his early career. Hearns and Arguello demonstrate good power because they rotate their hips and transfer the weight properly with the finished right hand punch on the left side.

Quite a few fighters could generate more power if they knew the importance of proper stance, hip rotation and weight transfer. Also balance is key to maximizing power in punches.

 

Proper technique is HUGE when wanting to punch hard. Consider that Hearns wasn't much of a puncher in the amatuers and suddenly became a killer in the pro ranks.

Why?

Technique.

Of course, he had to have other attributes there too but proper technique is the biggest difference a fighter can make to maximise their power.