How do i tell my step daughter shes perfect the way she is?

my step daughter is 16 abd get a long great for being 5 years apart. We shop together at the same stores, have a couple of the same interests and I even rescue her at school when her period comes and she can't deal with it. (Yes I know its wrong but she's a straight A student).

But how do I get tell her to stop comparing her body in a negative way? She drops settle hints like "why are you slim but wear a size 8?" Granted I'm 5'11 and I'm Greek and Persian and she's only 16, 5'4, size 4and the normal American girl (kinda looks like Christina Ricci). She second guesses herself when we go out eating now. (Like if I order a hamburger and a side salad and a coke, she'll order a salad, only lettuce, carrot with no dressing and water). I'm worried of their is something going on that's wrong!! I've tried talking to her and I get nothing. So I drop it, I've asked her mom, her brother, her father and they tell me nothing too!

How do I strike up the conversation where I'm not pushing or tell her its OK, not to look perfect. She some reason feels the need to Miss Perfect and mentions Karly Kloss and how slim she is! I just need some advice, please!