How do I politely tell my girlfriend to lose weight because her face is so beautiful and she would look even better if she were slimmer?

I understand that she is stressed at work and she doesn’t have enough energy to work out. She has a more demanding job than mine and she can’t exercise as much as I do. I like her as she is, but I know what she looked like when she was slimmer and I would be more attracted to her. It’s not like I want to change her, because that’s something she can work on

How do I politely tell my girlfriend to lose weight because her face is so beautiful and she would look even better if she were slimmer?
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