I don't think it's necessarily a question of size, certain exceptions aside -- no one ever looks good in spandex bodystockings, and when there's not a lot of clothes involved, the outfit typically won't be flattering on someone heavy. More than that, or cosplaying a character the same size as you, is knowing how to handle your size with the costume you are going for. Tailoring and fit are a HUGE factor, no matter if you're eighty pounds or two hundred and eighty.
Of course, being heavier, I'm naturally biased on this subject, but the blanket statement of "cosplay characters the same size as you" drives me mildly nuts. There are times that it matters, and times when it is irrelevant, and done right, it can be rendered immaterial in most situations unless -- as with things like eye colour -- it is a MAJOR part of the character. But if your costume is too tight or too loose, it won't look good no matter what your build is.
