The personality types associated with the body types have been pretty much debunked, but the body types themselves are pretty solid, and used pretty widely throughout the fitness and nutrition 'industries'. While most people aren't a pure Xmorph, they usually tend toward one of the three. There are certain things associated with the types, like variety of exercise or food that is most effective, that are fairly solid and based in modern research.
You'll almost never see a really skinny guy with big hips and a wide frame, the same with a fat guy with a very small bone structure. The most athletic people all tend to share very similar qualities with each other: wide shoulders, narrow hips, build muscle and lose fat easily, etc. It is probably as simple as the genes that make a person stocky and wide being linked to the genes that make them gain weight easier, or build large muscles. Likewise a very skinny person will have genes linking his slight frame to a lack of muscle and a fast metabolism/less efficient digestive system.
I can stand next to two of my brothers and pretty much make the body type chart, so personally I think the types have a solid bit of fact to them.
