It is the application of your body weight; getting as much of it in the right place before impact. Being bulky doesn't assist that; I don't know, necessarily, that it hurts it either. I either case the whole key to punching hard is punching properly, technique-wise. I remember watching a former Mr Olympia box, on ESPN, against Dickie Ryan (a guy that ended up having a ton of fights). Mr Olympia came out swinging and seemed to be hitting pretty hard but those big arms got heavy and he got tired of swinging them. Ryan rode out the first and stopped him in the second with a left hand to the body.
The guy on expertboxing came from the gym in LaHabra, California. That used to a good gym; back in the 80s, pros like Rocky Garcia and Baby Joe Ruelas trained there. Freddie Roach taught boxing there for a brief time when he first went out on his home; my little brother's cellie trained with him before going to prison.