He says he "gravitated towards thinner women", but this is his first "serious romantic relationship", so I assume this is the first woman who gave him enough attention and stuck around long enough and gave him sex (probably lost his virginity to her), so he thinks this is the best he can get. I mean, he's writing to an advice column, NPR no less. He's dating her out of desperation, he's one of those pigfuckers. She was fat when they started dating, because that's all he could get, and now he's realizing fat gets in the way of everything and wants her to stop being fat. The chances of that happening are about 0%.