A lot of toddlers actually have very little appetite when they are not going through a growth spurt. This is definitely true of our 2-year old, even though he has incredible energy. He could eat as much as he wants but he doesn't want to. Of course most of the foods he is offered are healthy. If he had a chance to eat unlimited sugar or junk food, maybe things would be different.
I think that kids who are extremely overweight probably have a genetic predisposition toward overeating or inactivity or have been allowed to eat too much junk food. As I mentioned, some foods that are considered healthy, such as juice, may not actually be so good for kids because of their high sugar content.
Having grown up in a family in which we were not fed enough, I am suspicious of the motivations of people who put kids on diets. It is also true that some adults with the absolute worst weight problems were started on diets as kids and it seemed to make things worse in the long run.