I do have skinny friends and when they say things like that to me I usually just brush it off with a "you're fine, you're gorgeous, don't worry about it". Incidentally, that's the same kind of response I get if I'm stressing about my weight. It bothers me for the same reason it bothers them (even if they really are at a healthy weight). Good friends don't do it to be hurtful, everyone has their own insecurities for their own reasons and if you're friends they just...come out. Sure, some people do it because they're fishing for compliments but I really believe
good friends just do it because they, skinny or not, have their moments where they feel like a cow.
My sister is 18, size 4, 5'2" and a former dancer/cheerleader. She's gorgeous. She's the tiny little thing that's always been the "little one" and got teased as a kid because she was
so tiny. I'm sure she could go into any store in the world and pull something off the rack and have it look great. She's just one of those people. Occasionally, yes she makes some random remark about being "fat" when she's just pigged out on McDonald's and feels bloated or something...but I know it's never meant to be her insinuating anything about me. She's the person I take shopping with me because she never sees my weight as an issue. The way she acts with me, it's like she doesn't even notice that I'm overweight. I wouldn't take what your friend said too personally. I'm sure she doesn't see you as "fat" at all and was probably just feeling really stuffed after a big meal.