Most Helpful Girl
They don't, lol. In your age group, most guys [and people in general] tend to have a herd mentality. They'll openly like what is socially acceptable for them to like, and not admit to or go after anything that isn't 'the norm' for others their age to like. Which means 'bigger girls are unattractive/haha fatties'. That doesn't mean that's what all of them think, but it does mean that the ones who do like bigger girls are usually too immature and/or insecure to not care what anyone else will think of them for it. That's high school for ya. The older you get, the more guys who like bigger girls seem to pop out of the woodwork. It sucks now, but you'll get through it and you'll find someone of quality who is attracted to you exactly as you are.
I'm bigger and guys never showed any interest in me when I was younger and hanging primarily around people in my own age group. This changed somewhat over the years. I've been with guys who were attracted to me sexually, and now I have a boyfriend who loves and is turned on by my body AND mind. In the past several months, I've noticed more and more guys who are attracted to bigger girls, or at least not so discriminant against them.
It'll happen, I promise. It might happen when you least expect it, like it did for me. But don't get too down on yourself, just focus on other things and enhance other areas of your life. It's attractive when a girl has things going for herself, when she has her own interests and hobbies, her own life to worry about. People pick up on the 'need a man to be happy' vibe some women give off, and it tends to take away from their attractiveness, the same way blatant lack of confidence does, because it gives the impression that she doesn't know how to just be her own person and be okay with it.