The reason fat people aren't jolly any more is they all read the article in the July 2006 article of the Archives of General Psychiatry that said they were supposed to be depressed and moody.
http://www.msnbc.msn.com/id/13691328/ns/health-mental_health/t/fat-people-not-more-jolly-says-study/#.T9zYs_WSngw
What's more interesting to me is where the idea that fat people were supposed to be jolly originated. I have a theory that it's rooted in the archaic theories of humorism and their relation to temperaments.
http://en.wikipedia.org/wiki/Humorism
http://en.wikipedia.org/wiki/Four_Temperaments