The trouble is just that people are really good at taking a cause and effect like "laziness makes you fat", then flipping that around to if you are fat you are lazy. I suspect the problem is just that it's true X percent of the time, where X is high enough that people feel confident in the false connection.

Underlying motivation is just that we like feeling better than other people, it's not that surprising. It sucks, but we're not particularly logical or rational beings :-)