F=MA Force equals mass times acceleration.
*haven't even watched the Kai Greene video, BTW*
If a guy Wayne 260 can accelerate his fist (or his body, his fist is just an extension of his body arm shoulder Etc) at a rate similar to a guy who weighs 160, obviously he is going to hit harder.
Technique and body mechanics, whether it's natural or trained, is always much more important than size and strength. Rotating your hips and driving that Fist from the ball of your foot will increase the "mass" behind the punch, compared to just throwing an "arm punch", where the only "Mass" behind the punch is coming from the shoulders N arms. Compare the punch from a guy putting the majority of his body weight on his back foot, hell, you could even have the front foot off the ground (as in the case with Bruce Lee), to a punch where his weight is mostly on his front foot (or even 50/50), and the punch with the body weight on the back foot is always going to be much more forceful.
Add to that having to weaving multiple punches together and having each punch be thrown "from the hip". That's where technique/training separate amateurs just hitting the bag for fun or cardio and guys whose income depends on it. Throwing a 3 4 or 5 punch combo is a whole lot different from throwing a single overhand right.
If kai's hitting a bag, I doubt he's gonna expect anybody to be super-impressed.
