I'm wondering where it came from, and it occurs to me that probably in earlier eras, when plentiful and nutritious food was harder to come by, maybe fatness was seen as a positive, an asset. Maybe fat people were assumed to be jolly because they were in fact happy. They had attained an enviable degree of material success, and the evidence of it (the weight) was not despised either as a symbol or as itself. What's not to be jolly about?