How can I make her believe she's not fat?

ok so every once in a while ill be laying down with my girlfriend and ill rub my hand over her stomach or message her back, and shell tell me to stop cause she thinks she's fat. its not like she complains about it a lot but from time to time she brings it up and it honestly makes me so mad. because she isn't over weight at all, and I'm not just saying that because I'm her boyfriend. like sure she isn't super skinny but definitely no where close to being fat. I personally think girls that are super skinny to the point where you can see there ribs isn't attractive at all I like it when a girl has some curves. I've told her multiple time that she is not fat and that I think she's beautiful just the way she is but I know she doesn't' believe me. so girls is there anything I could do to make her really believe that she isn't fat?
How can I make her believe she's not fat?
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