That's just what I'm getting to in my next batch of notes. You see, it all depends on the style. For cartoonish looks, they lower or higher the head count to make a person look chubbier or thinner. In reality, the head count probably won't change as much as in the cartoons. The count would still probably stay between 6-8 heads (I think there might even be 9-head-tall people out there. You just never know).
But like FTM said when he spotted your oversight, don't hesitate to use references. Take pictures of people with different structures, or if you don't want to seem so nosy, google pictures of people, or... go to the stock accounts in DA. They might have stock photos of different body types you can use to practice on.
Also, as you practice off of real life, take a look at how other artists do it. Take a look at their many drawings of body types and derive notes off of them. Humans learn from each other, it's a natural process.
You know, I seem to know what to say, yet I never think to do what I suggest until I say it. Weird, huh? I think I'll take that stock advise of mine. There has to be some good photos out there to practice on.
Wait... What were we talking about?