I was overweight when I was younger and then I got put on medication which acted as a metabolic stimulant and an appetite suppressant because of its side effects. I lost a lot of weight and I started to feel less insecure about myself, but then a couple of months later my insides starting talking and were like, "HAY GAIZ, I THINK IT IZ TIME FOR PUBERTIZ". I wish I'd hit puberty much later in my adolescence. I was hardly eleven years old when it happened and the moment I became sexually aware, the more I began to realize how much my body felt "wrong". I don't think I can relate to you with trying to compete with other girls, because I knew if I wanted to, I could've. Most girls would kill to have porcelain skin, be cute and petite, and still have the sex appeal of a larger chest. I suppose this made me despise myself even more, because I felt like I should be happy with my body, but instead, before I even had the words for it, I wanted to hide it from the world and wished I was built more like a boy. Before puberty, it was much harder to tell the difference and much easier for me not to associate myself with a gender. I'd always done more "masculine" things as a child without being disparaged for it, but was never really told to be that way; basically, I'd never been forced into a conflict with my gender identity before this time. I think to some degree it's harder knowing that there isn't much else wrong with your body other than the fact than it's attached to you. There's a lot of times I wish some "cis-girl" could have had it, because she probably would have appreciated it a lot more than I ever had.
I don't think wanting to be skinnier is necessarily gender based though. There's a lot more to envy about girls than their stomachs and thighs. Same goes for the stomachs and chests on men. For instance, if I were cis-girl, I would likely find the time to envy a girl's chest as well, unless I was already buxom enough. >_< If this a scenario of purely weight issues, it's likely that you'll be at least slightly envious of anyone who is skinnier and or "more fit" than you are.