Overweight people are overweight because they eat a lot.
This is generally true, but there are many, many exceptions.
Obese teens may actually have a LOWER caloric intake than their
thin counterparts. A common occurance is the cycle of obesity. When
teens are obese, they are often embarrased of their appearance and
tire easily, so they exercise much less than normal teens. Even if
their caloric intake is very low, they still would not lose any
weight. Some overweight teens do eat little, but because they do
not exercise at all, they remain obese.