Bourne Identity he buffed up a lot. Then again, no one thought RDJr would play a good Tony Stark, and he buffed up. "Gotta disagree here. From all the accounts I read back then, casting RDJ as a party-boy drunk millionaire was pretty well-received - if it weren't for the iron suit, the joke went, it might've been confused for a documentary.
As far as casting Damon as Steve Rogers, I'm not feeling it. He's a fun actor, yes, but I don't see him having the kind of presence as Cap to order around Thor or Iron Man. Damon would, however, make a hell of a Winter Soldier.