she's very happy, well integrated and academically excelling.
If this is true, leave her in school, please. It's her security,
Possibly express concern for her health and arrange a consultation with a dietician and appointments as necessary. This is for her, privately, without you.
Then - you need to butt out. How dare you criticise a child you've sent off to live alone, for what she does or doesn't do? How dare you? How dare you criticise her body? Her actual body, that she lives with every day (unlike you. She's not with you every day, is she?)
You don't want a 'fat' daughter, that shines through your post. Treat her respectfully, and support her rather than criticising (and this includes a lot of 'keeping your opinions to yourself', in present circumstances) and it will all be fine. Women want to look the part society dictates, so she'll do the thing if you give her space.
Why not ensure every moment you have together is a joy, without criticism? (Realistically, you have a teenage - I hope -daughter, there's going to be some pain...)