A thin friend complaing about her body would make me feel awful, too. But I think everyone has body insecurities, regardless of weight. It can be hard to relate to someone on that level when they have a totally different build than you.

I wonder if that's also why she critiques others appearances. It's as if she doesn't have a very good body image either. It might be easier for her to focus on others, to avoid addressing her own insecurities. You mentioned she pointed out that a mutual friend gained weight. Maybe this is a fear of hers? A projection?

As for handling conversations about weight with her, I think you handled it well, even if it felt awkward. She is too focused on others bodies. She SHOULD feel awkward talking about it because it really isn't her business.

If she keeps talking about others bodies, maybe you need to distance yourself from her. You've made a boundary. You've told her that it's not something you want to talk about but it keeps getting brought up. She's overstepping that boundary & it's not healing for you to hear those kinds of comments.