This has been proven in many studies and articles, and it is logically sound. When men are obese, much of their fat buries their penile shaft, so it always presents an illusion that they have short cocks. Because of the fat around that area, the actual usable length is also diminished. Only the length that can be inside the woman's vagina should count towards the length.
In my case, when I last measured, I was 203 lbs. The chubby asian dude that I was measured 5.5" fully erect.
Last night, I got curious. I remembered all those times when my GF told me I had the most beautiful cock she had ever seen. So I took out my ruler again. Keep in mind I now weigh 169 lbs, and my stomach is virtually flat. Final measurement: 6.25"!!
This wasn't an ego thing, but a nice thing to know for guys who drink beer and have a big ol' belly. People tell you that there's nothing you can do to have a longer penis. Well, I guess that's not entirely true.