Why are thin people not fat?

There are many reasons why thin people are not always considered to be obese, and sometimes even considered to be "fit." For one thing, thin people tend to have a lower percentage of body fat than obese people. In addition, a greater percentage of lean muscle mass means that a person's body is burning more calories even when they are at rest. Finally, thin people typically have a lower resting metabolic rate than obese people, meaning they burn fewer calories even when they are at rest.