Superman has alien muscles so he shouldn't look too buff.
I thought the swimmer look that Singer chose fitted the character better, Superman sort of swims in the sky...
...
*shudder*
Batman is a human so to do all the acrobatic things he does he needs to be buff.
So arguably it makes more sense if Batman looks more pumped up than Superman but the falsies... I've never liked them since Keaton and I don't like them now.