Though there's no peer reviewed study for this (proving in the first place that they do have more daughters on average, I mean) this could possibly be correlated to the fact that excess fat completely alters hormones in men. They are in the end the carriers of the Y chromosome needed for a boy, and odds are that Y chromosome, which thrives in higher amounts of testosterone for obvious reasons, doesn't do well in a fat body with a lot of hormones that mimic female hormones. This isn't verifiable fact, just an educated guess