http://en.wikipedia.org/wiki/Four_temperaments
After you read that read some shakespeare(I reccomend caesar, especially the part where he mentions he doesn't want skinny people working for him because he doesn't trust them, and that he wants fat people; that's a reference to temperments/ humours/ constitutions) or nearly any other form of literature from a couple hundred years ago back.
Pay attention to the faces and notice that fat people are classified as one, and skinny people as another, with others in between.
I'm sure you recognize that Myers cognitive function types on the following page:
http://en.wikipedia.org/wiki/Four_humours
Fat and skinny has to do with a personality balance, supposedly, and probably also because skinny people are skinny(which alot of people don't like) and fat people are fat(which alot of people don't like) so they find eachother like a game of elimination.
I'm sure in your lifetime you have noticed the boistorous fat man. That's an example of this.