My gf is not fat, but she's a bit chunky. She just bought a bikini for the spring that's not flattering. I'm not sure if I should say something or even how to do that. I'm not trying to fat shame, but ...
Only comment if she asks you, and then be VERY, VERY careful with what you say and how you say it. "How do I look?" is a scary question for a guy who wants to stay alive. :-)