I called my girlfriend fat?

My girlfriend has a really slim upper body and she has a bigger bum and thicker thighs there not that big but she thinks they are. She does complain about them but I don't see the issue. So this afternoon i was sitting on your couch and she came in and lay next to me and but her legs over me and I jokingly said "get your big fat legs off me" and stared to laugh she got really upset and starting crying say that I'm so horrible and she hates me, I really didn't mean it in a bad way I was only jokeing with her. She won't speak to me. what can I do to make her forgive me?
I called my girlfriend fat?
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