Given the "Gals want to know" thread, I felt it appropriate to provide da facts. This is such an important issue that the Editor-in-chief of Men's Health, David Zinczenko wrote it!

Size Matters - How Weight Can Affect Your Penis Size

As much as women dream of having a smaller waist, so does a man wish for a larger penus. We're a culture obsessed with physical appearances. We'll do just about anything to fix what we don't like, whether that's going under the knife, trying out the latest fad diet, or hopping on the hottest contraptions. So, what if someone were to tell you that it may be easier than you think (the penus thing that is)?
 
Being overweight can make the penis appear smaller than it actually is. If you consider the ratio between girth and penis size this fact makes a lot of sense. By carrying around all that extra weight you could inadvertently be covering up a good portion of your equipment.
 
Dr. Mehmet Oz, made famous for his regular appearances on Oprah and known particularly for putting poop on the forefront of Americans minds, says that for every 15 pounds of fat lost a man can expect to perceive increases of more than an inch in penis length.
 
So, does fatter really mean smaller? The powers of suggestion are probably enough in this case, but let's get the facts shall we?
 
The length of an average man's penis is about 3 inches when flaccid (yes, that's true), but the fatter he is the smaller he can look because the fat at the base of a man's abdomen covers up the foundation of the shaft, making him look smaller than he actually is. So, by decreasing the amount of fat on the body you enable more of your member to be viewed, which actively "lengthens" the penis. Losing up to 15 pounds of fat can increase the look of a penis by half an inch.  

I'm sure you've all heard of the hung skinny man. Though there is always the case of a man truly having an unusually large penis there is always the body to penus ratio to consider. On a skinnier man you are seeing his entire shaft all the way to the base, but he also has another thing on his side. He's skinny, so his penis in comparison to his size is going to appear larger.  

When it comes to a man and his privates, fat is his body's side-view mirror: Objects appear smaller than actual size.  

Penises are like boobs, some people have large ones some people have small ones. I don't know about you, but when faced with a 3-inch penis, half an inch can make all the difference.