According to an article in Cosmo the lady in Big Fat Fabulous life does have a condition that caused the weight gain.
"Thore was thin growing up, but she still suffered from eating disorders and struggled with her body image. When she got to college, she was diagnosed with Polycystic Ovarian Syndrome. She gained 200 pounds during those four years, and they've stuck with her, since her condition makes it hard to lose weight."