I honestly can't say. I have known of very overweight brilliant people and I have known some really brilliant thin people too. I think we tend to think thin=smart though. It's defiitely the way it is portrayed in the media for certain. How many movies have I seen where the brilliant, thin and fit woman had some high profile, glam job. Likewise, how many times have I seen overweight characters (even in cartoons) being portrayed as slovenly, low-performing, stupid or ignorant.
Luckily, there is real life though. Overweight means you are overweight and nothing more or less. Being thin and fit also means you are thin and fit--nothing more and nothing less.
"Be who you are, say what you feel. Those who matter, don't mind. Those who mind, don't matter." --Dr. Seuss