Lots of people look small when they're near tall men.
Remember Bale? He was so thin in The Machinist. But he had an awesome shape as Batman. So it does not mean anything.
Henry seemed skin too in some movies. Look at "Mon-El" : he looked like a fat tall giant, making Superman smaller near him. Same when Superman is near Zod because Shannon is taller. While Henry is not "petite"...
I think Matt can do it perfectly. And remember Henry had to work out to become a healthy Superman. Matt could do it too.
And as far as I know, lots of people are convinced Matt can be "Christian Grey" who is a "dark" character. And it's not only because of his physical (which would be perfect to play Bruce Wayne as a playboy) but also thanks to his talent. Lots of people believe he has this "dark side". (anyway he's an actor, so he can play a character, whatever if this character is a happy one or not ; lots of actors are said to be the next Batman and they haven't necessarily played a dark character before)
(noboby need to be "dark", even Bale was not so much, it was mostly because of the dark light and his cotume + of course, his talent : he was depressive, and Matt can do it too according to some scenes in White Collar)
But I can be ok with Batman looking less muscled than Superman. I mean, Superman is obviously the stronger, so...
I've always thought of Bruce Wayne as a playboy, not a super-muscled man who seems so strong he could participate in a gladiator tournament
(cf MMA). So I can't imagine Bruce portrayed by someone else. Matt Bomer and his suits are the perfection, the kind of man I would see partying with girls. (and Matt does it sooooo right!!)
I don't understand why you're all talking about the muscles an actor should have to be Batman. (should I recall nobody payed so much attention to that for the previous movies? And it was believable all the same -ok, I did think Keaton was horrible, even if he's a good actor, I could not help thinking he's not good looking at all and yeah, you can say HE was quite skinny as Batman, but the costume modified his looks and he looked strong all the same, so...who cares?) Some people seem to forget Bat is not only a fighter, he is also a man. A really really pretty man, very attractive. I used to dreams of him xD
Go Matt! GO!!