I was thinking defense too. You don't have to be big to play defense, just fast enough and strong enough to run/take the other guy down.
If we're talking about a league of athletic clones, where the guys on offense have the same athletic skills as the guys on defense, same attributes, same everything, then it sounds a lot like rugby to me. But, rather than a field full of big lumbering guys, I want to see what a bunch of stocky, quick guys can do. More exciting, I'd venture.