On the other hand, Republicans are more likely to be married. As comedian Emo noted back in the 1980s: "My sister and her husband just found out they haven't been legally married for the last ten years because the minister who married them was a fraud. It's really sad. Now, she'll have to lose all that weight."
So, it's hard to say for sure. There's probably something in the GSS about this. Joseph Fried's book on Republicans and Democrats says that the weight of evidence suggests that Democrats are a little fatter, but it's probably a pretty close run thing.