[_greim_]

The article claims two competing theories of obesity: the energy-balance and the carbohydrate-insulin model. The energy-balance theory isn't much of a puzzle, as you say. The puzzle is rather how to reconcile these two theories. According to the article, fructose is the unifying factor. Eating more fructose drives increased appetite and lowers metabolism. This feeds into the energy-balance mechanism, causing obesity. To your point, the higher appetites caused by fructose are incentivizing restaurants to offer larger portion sizes.