I've no doubt Affleck will fill the suit out very well. He's a very broad shouldered guy, and broad shoulders + height makes a person look quite strong and intimidating. As impressive as Bale's muscle gain for BB was, he never really looked 'big' overall or intimidating - in fact, I can't really think of any previous Batman actors who looked that way.
The Batman films have tended to rely more on his presence/voice/fighting skills/etc to have him intimidate the bad guys. Not many have approached the issue of his sheer physical size. Many of the comics have Batman written as a very large and strong male; I think it was The Dark Knight Returns which made a specific reference to his size.