My guess would be that there are 2 factors at work:
1) Single women have a greater investment in maintaining their figures in order to maximize their attractiveness. Or maybe it's more accurate to say that women who have settled down stop caring about a few extra pounds as long as their partner still finds them attractive.
2) It's a drag to cook for one, but it can be very rewarding to cook for an appreciative audience. I spent many years cooking for myself, but eventually I began eating more meals in restaurants, and then all of them. Cooking for myself was just too much trouble. But when a new boyfriend entered the scene, my interest in cooking would rekindle.
And, while restaurant portions are big, most home recipes are portioned for 4 people. The cook may intend to save half the pot to freeze for another meal, but when 2 hungry people sit down to a delicious meal, and there's more right there on the table.... Well, that food in the middle of the table is already paid for, and it always seems that just a little bit more won't hurt.
Is there a similar study for men? Or is science only concerned about the size of women's bottoms?
I would expect, from my own experience, that no small part of it is that partnered women eat more meals with other people. When I eat alone, I eat a lot less -- I often skip meals (I hesitate to say that because the social pressure not to skip meals is so intense) or only have a very small amount of food because I'm not really all that hungry. When I eat with others, I tend to eat more, and more often, on somebody else's schedule rather than my body's.
And portions. When we eat at home, my husband tends to make up two plates of food with equal amounts on them, as if we were the same size. He doesn't want to be one of those jerks who is always telling his wife she needs to lose weight, after all. In restaurants it is nearly impossible to find reasonable portion sizes.
I especially notice this because my own caloric requirements are way, way, way less than average, even for my size. Because I track my calories, the pattern is pretty obvious, especially when we eat out. But obviously regularly eating just a bit more than you really should will add up.
Like you, I'd love to see a broader study that looks at somebody other than women. My husband gained weight after we moved in together, too, after all, and my suspicion is that it was for the same reason. It is interesting to see a correlation between social factors and weight actually proven. I'll need to read that study in more detail.