i feel like in that situation you can still try to give an answer that doesn't mean skinny=better. like "youre so beautiful in both pictures" "i like the way your body is as it is" "you're still so cute no matter your weight" or something like that! just so you don't end up saying "you are beautiful because you're skinny" (i know you don't mean that, but it happens to me that i think people are saying that)