You can get fruits and fruit juices in your winter months, that is newer. OJ came in small glasses because it too was a luxury.

But HN linked an article a few years back about some research indicating that in the 1980s people ate as many or more callories, and got on average less physical activity, but were significantly less overweight. They reaction was that it was wrong, because thermodynamics, but the thermodynamic argument is a vast oversimplification. We can and do excrete plenty of energy such that other organisms post process our waste. Fat in particular can go unused with little notice. We can burn or conserve energy in ways that are hard to measure by just observing behavior. Given that complexity, it wouldn't surprise me that chronic inflammation could be a culprit. It could be the removal of a burden too, such that we are more efficient at processing calories. The food equivalent of meth amphetamine being available everywhere and cheaply doesn't help either, but enough people are studying it that I don't think it will be one simple thing.