If I say "ugh, I'm so fat" to a guy, I certainly appreciate a "hey, I think you're hot" in return.
But if I don't say anything of the sort (and I try pretty hard not to), if I'm just going about my day tra la la like I'm a normal person... it really weirds me out when guys come out of nowhere with "you know, I actually like your body" or "I don't mind a curvy girl one bit." I mean, gosh, thanks.
Hint: statements like that translate into "I assume you think you're ugly, and I assume a lot of people agree, and I was just now thinking about how generally-considered-ugly you are, but hey, I'm willing to tolerate your flaws, aren't you lucky?"
If you want to compliment a fat girl, tell her that she's pretty. Don't tell her that she's pretty anyway.