I cover this in depth in my books. But for now...real simple.
Heavy training hits the fast twitch Type 11 muscles and they are the bigger muscle fibers. They're designed to burden heavy weight for short amount of time. Hence, it makes sense to train heavy and build the bigger muscle.
One problem -- not everybody has an abundance of them.
And if you don't, the stress will just go to the joints and tendons.
But ALL muscles can be pumped. And pumping will stimulate more mytochoria, which in turn creates the ability to regenerate more muscle. Pumping fills the muscle will nutrients and essentially "shapes" it, creating the "look" of a larger muscle.
Serge Nubret trained in a very slow, strict style using relatively light weight i the 12-15 rep range. Seemed to work for him.
http://4.bp.blogspot.com/_v4BnOvWsCM0/TAK1AjLRfXI/AAAAAAAAA5A/YpJ2BS0IyY0/s640/Serge+Nubret.jpg
Flex Wheeler was a strict form light guy too. VInce Taylor and Paul Dilett too. Even Mike Menzter -- yes the guy who promoted "heavy Duty" training. That was a total gimmick/intellectual experiment. He never actually trained that way. In fact, by the time he promoted it, he was horribly out of shape at age 40.
Also, keep in mind, "light" is relative. What I'm referring to is not a 6-8 max to failure range but something that will create a pump. It depends on the individual.
That's PART of the answer.