How do you encourage a gf's weight loss without making it sound like you think she's fat?

My girlfriend isn't fat. is she a little bit thicker? yea. she has big boobs and a nice big ass and she may not be petite but I think its sexier.

however, she's recently been starting to work out more and is really trying to lose weight. I want to encourage her and support her, but I feel like if I do it seems like I'm telling her she's fat. Is there a better way to encourage and support her weightloss without making it sound like I think she needs it?
How do you encourage a gf's weight loss without making it sound like you think she's fat?
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