I've heard that part of it is not muscle mass or tone, but points of attachment -- leverage. As I understand it, chimp muscles are attached further up from the joints -- which gives the muscles better leverage. This trades strength for speed of movement. You can outjab a chimp, but once he gets hold of you, he'll tear your head off. Also, as pointed out by another poster, they're disproportionately large above the waist in comparison to humans. A chimp's forearms are huge.
That's what I was thinking I'd heard too, that there were points of physics involved, in addition to connective tissue and such. I wonder what competitive advantage came from us becoming weaker. Perhaps the fine movements of the hands meant a need for sacrificing strength. Who knows.