A lot of it has to do with the physics applied behind their punching form and the manner they choose to throw. They are all also out of this world type fast. Factor in that speed with how their limbs are close, and compact as in tight to their body. Most of the power in a punch starts from the ground, up through the hamstrings, glutes, hips and then the core onto the arm and then the end point which is the fist. Because their limbs are usually shorter, they are closer to the origin of the torque. Meaning, the max power of the punch is just before the end of its extension. These guys know how to apply proper form for their body type, combine it with speed and torque and timing, for maximum effectiveness.