According to Vicky Belo, a famous cosmetic surgeon in the Philippines, during a radio interview where she answered questions from callers to that show,
a fat man has a smaller penis.
This is not only due to the proportion of a fat man's body to his penis but also because an over weight body has more estrogen (female hormones) than a normal, leaner man's body. This is supported by other studies where a man with more muscles (a healthier, more fit man) has more testosterone (male hormones) in his system.
Cosmopolitan magazine however does not take "pumped up testosterone" light - it's not always a win-win situation. Just because a man has a better body, has more testosterone, doesn't mean he's more "desirable". Also according to studies, men with higher testosterone levels are more likely to cheat - blame it on their testosterone levels and lack of self discipline. However, just because there is such a tendency, it doesn't make it normal and uncontrollable.