Beach Patrol, I am 5'7.

I've definitely had those thoughts about people, and still do sometimes. I think the important thing is to not let it define your friendship. Try to keep in mind how you felt always being the bigger one of your friends, and remember that whichever of your friends is gaining weight, that's how they're feeling now. Being overweight isn't fun for anyone, and we all know what it can do to your self-esteem and how you treat yourself. Don't forget this when you're interacting with her, and if it seems to you that she has a realy problem with her weight, let her know that you're there to give her a hand if she ever needs it. Your friend's well being is more important than her size compared to yours.
I've been the skinny one in a groups of friends before, and I used to often compare myself with my larger friends to feel better about myself. Now that I know what it feels like to be overweight, though, I don't think I can ever do that again. It's more important to me that all my friends are happy and healthy than that I'm skinnier than them.

I sooooo admire tall people!!! (You're tall to me! )