Most Helpful Girl
I was a chubby girl myself. When boyfriend & I started dating, I was heavier then he was. He was skinny but taller than me. I was not obese but was quite chunky. At first, he wasn't into me. However, ever since high school, I've always had a big crush on him. Yes, he was skinny but he didn't look like a weakling at all. I never saw that way at all. He's "grown" a lot bigger now while I've slimmed down to "sexy levels" (as he would put it) now. We're still together and still so much in love with each other.
Anyway, maybe it's me. Maybe, I was unlike the other chubby girls. I think they're insecure about themselves and their "defense mechanisms" kick in automatically in anticipation of a possible bad first impression from a guy physically very opposite. It also might be that they think they're doomed to stay chubby forever because, deep inside, they refuse to try hard enough to get more fit.
So, don't feel bad about yourself. Don't feel sorry for them either. They've "type-casted" themselves on a path that a guy like you cannot reach.
Most Helpful Guy
I'd guess they don't think you'd be interested. Talk to them, see if they appreciate your interest in them.