The power from the Donaire punch came with how much of his body he put into it. He rolled Montiel's right hand and used that momentum to counter with a hook. Another thing that translates to power is good form in the shot. Wide punches means a loss in power.
They could probably hit decently hard if they were trained to do so. Unfortunately the amateur system rewards quick, pulled punches. You make that your style and it's hard to throw hard. Look at guys like John Molina. Not a lot of amateur background, but a hard hitter. He's trained to hit hard, not to tap you and score points. A lot of it is timing as well. If your style is based on swarming and volume punching and speed, you can't pick your shots as well. If you're a puncher, you aren't usually a whirlwind of action. You look for the right time to land the big shot. IMO it's a question of style, not body-type.
Exactly , Hearn use to hit with feather punches until Stewart taught him how to use leverage. Roger Mayweather was another fighter who didn't have a lot of power until master the art of leverage. some people have natural gift for body mechanics, meaning they now how to get the most torque from their body, having lots of muscle does not alway translate into punching power but having good technique can.