Why is my girlfriend beating herself up?

My girlfriend keeps packing on the pounds. I the past 2 months, she gained 35 lbs. She comes to me and cries on my shoulder and I reassure her that it's ok and that I love her for who she is. I actually prefer bigger women. I don't tell her that though. I am very cool with it and she is ashamed of herself. It seems every day there is a different pair of jeans that doesn't fit her anymore and she just cries and cries. I just try to keep her calm and tell her how much I love her. What do I do? As I said, if I had my choice, she would continue to gain weight, but that's not why I date her. I just want her to be happy no matter what though.