Yes, I'm that girl, and quite frankly it gets annoying.
I may not look it, but I eat all of the time. Getting questioned "do you eat?" 24/7 is really annoying. Yes, I do eat, I just can't gain weight. I constantly eat food, but you'll never be able to tell. Food is my favorite thing, but you're so quick to judge. Just because I may not be 200 pounds, or even 150, does not mean I don’t eat the way I'm supposed to.
Just like the obese, the skinny get offended by your annoying remarks too. Not everybody is made to be perfect, we're all made individually as our own person. So, before you ask that 100 pound girl if she eats, maybe take into consideration that she is just as annoyed as you that she cannot gain weight.
I do eat, and I do have meat on my bones, it's just not enough for your liking, and for that, I will never apologize. It's just as hard finding clothes being a 100 pound eighteen year old, as it is being 300 pounds. It’s no where near fun, and the remarks get old, fast.
Truthfully, nobody is perfect, and you're probably not as perfect as you think you are, make sure to think about others feelings first. We are human too, believe it or not. We try to eat more, just to gain weight-- bet you'd never believe that. Treat everybody equally and the way you'd want to be treated, because we didn't choose this life, this life chose us.