He asked a question that can't be answered with the information provided, so my answer is as valid as any. Flip, but valid.
To say everyone who is in a happy, stable relationship who gains weight is a lazy fuck is just another flip answer. She very well might be conflicted about being involved with someone who isn't a long term prospect, how the fuck do any of us know the answer? The person in the best position to answer why Conundrum's short little gf chubbed up is him or her, that's it. Like I said, if he isn't in love with love her then he should either break up with her chubby ass and find a fit replacement, or shut the fuck up enjoy fucking her and only take her out to dark restaurants where his friends won't see him with his pet whale.
Problem solved.