How to encourage weight loss without sounding bad?

My girlfriend is trying to lose weight. I don't think she really needs to but she can, and she wants to

I want to encourage and support but I don't wanna sound like I'm ever calling her fat. How can I encourage her to keep up the weight loss (she uses weight watchers) without telling her she needs it or sounding like a d***?
How to encourage weight loss without sounding bad?
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