My[21f] girlfriend hates her body and I[24m] don't know what to do

My girlfriend has issues with her weight and appearance and I'm having issues with trying to reassure her and talk her through about why she feels this way, I've suggested going to the gym with her or changing diets but then she says she enjoys her weight because it makes other people mad and I don't know what to do anymore. It's becoming draining trying to help because it's being brought up every day almost and when I try to reassure her she just says "nah" or dismissive things like that and I'm just looking for advice