Why can guys 5'10 160lbs hit way harder than guys 7'0 300lbs ?

The answers simple: power is not just about size and conditioning its about technique. If a fighter is an arm puncher even if he weighs 300 lbs he won't hit hard because he isn't putting that weight behind his punches. If you look at the likes of Hearns and Jackson they put everything they had behind their right hands. Also accuracy, Sugar Ray Robinson could take out a top fighter with one punch but he admitted himself that he wasn't very powerful, he knocked fighters out because he was accurate. Btw: I liked your Youtube channel especially the Sonny Liston tribute.