Actually, some people are thin inside and fat outside, others are fat inside and thin outside, many are fat inside and out, and a few people are thin inside and out.
You can tell whether a person is thin inside by how much space he or she takes-up. People who are thin inside make room; people who are fat inside take room. For example, people who are fat inside walk down the middle of a store aisle, in a manner to obstruct passage. People who are thin inside keep to one side or to the other, to allow others to get past them.
Proportionately more of the people who are fat outside are fat inside; the fatness inside motivated the fatness outside.