after moving in with jamie you get the chance to snoop around, there you will find her statue, if you decide to switch it with a fake one she will start gaining weight
The problem is I didn't get to tamper with Jamie's statue in order for her to gain weight. I did snoop around her room(only got one chance to do so) but it said better not to mess with her statue and I didn't really see any other options which may helped to mess with it.
