While I don't encourage people to be overweight, you might even show her some art from the past, where overweight women were seen as beautiful, and how those paintings show women in a beautiful light.

One famous painting where the woman isn't fat, but she isn't too skinny, either (and not "molded" or muscular), and is shown as beautiful is "The Birth of Venus" by Botticelli. This is a famous painting of the Goddess of Love, and certainly, the Goddess of Love must be beautiful, yes? Here's a link to the painting and info on it (which I don't vouch for; anyone can add info on Wikipedia, so you can't be sure it's accurate, so keep that in mind) on Wikipedia: The Birth of Venus (Botticelli)