in my experience, girls in general dont want a guy who's overly fat, skinny OR muscular. they want it somewhere in the middle.
it probably sounds overconfident, but i dont care. every girl that has ever seen me topless (not that there has been a huge amount of them. probably 6 or so) has passed a comment at some point along the lines of "look at that guy! he's really muscular/skinny/whatever, it looks gross. not like you, you're perfect!" .
im not bulging with muscle, or skinny, but i am slim and well toned from top to bottom. in particular ive noticed that girls love guys with strong looking arms and core muscles. I've lost count of the number of times a chick has exclaimed at my back, which is apparently unusually muscular. my last ex used to hide my tshirt from me in the mornings so she could see me run around topless.
as a sort of a guide, i asked three girls if they thought christiano ronaldo (a footballer, for anyone who doesnt know) looked good topless. one said 'yes' straight up, and two said "ugh, no", with one of those saying "that looks kind of weird."