Some overweight people are that way though because of genetics, one of my closest friends goes to the gym, eats healthier than I do, same height yet is way bigger than me weight-wise. Most of her mother's side of the family are overweight too or even obese.
How long have you known her? Honestly if she is more attracted to you than you are to her, it will likely just end badly for her. But if she is fairly new to your group of friends, maybe after spending more time with her in a group you will know better what you want.