Is your friend very heavy, and likes everyone to be eating when she is eating similar to alcoholics like everyone drinking when they are drinking. If you are normal weight, which you say above, then just say something back next time like, “why are you so focused on how much I eat?” If her response is unfeeling say you would prefer she “doesn’t say anything anymore about it.” She can’t read your mind, say what you need or want. Especially if you are a female this is a lesson. Being polite, and letting people make you feel bad about yourself only goes so far, and then at some point you need to be self assured and “protect” yourself. This goes to communicating when you are uncomfortable with what someone says to you, when they touch you, when they stand too close, etc. even for men also, but men usually have less trouble speaking up. If you actually are very thin and in denial, she’s saying it because she is worried about it. She’s not doing it in the right way, but still worried. I have a friend who doesn’t eat much and she was anorexic for many years. She isn’t skinny anymore, but out at restaurants she still eats like a bird.