If your girl is putting on that kinda weight under the circumstances specified in the OP, you're not doing your job as her man. Now, I usually go hard on the bytches in my posts, but I gotta put this particular responsibility on us.
I had this EXACT same situation with my ex girlfriend. She gained hella weight after she moved in with me, the relationship got stale, and I stayed with her in that rut for a while. I was dissatisfied, but it was my fault. When we first met, I was the fukkin man in her eyes. She would have done anything for me, and would've probably harmed herself if I left her. I was the leader of the relationship, and it was fun and interesting. Later in the relationship, I took my foot off the gas, and started coasting. This is why she fell off so hard in both her looks and attitude.
You gotta be constantly upgrading yourself and your girl, or else one of two things will happen: 1.) She will fall way off, as previously described, and/or 2.) She will find that upgrade elsewhere, with the next nikka.