link != the type of relationship (Score 1)

Could it be just how a slim person spends his time verses how a fatter person might?

Yes, spot on. A link is merely a link and speaketh not as to what the exact relationship is. Does blubber make you dumber? Probably not. Does lack thereof make you smarter? Probably not. However, do body builders and fatter people alike tend to spend more time on their bodies (one way or another) than people that spend a majority of their time developing their minds? Probably so.