which humans did for millenia without getting too damn fat until about the 1990s

That's the really mysterious bit to me.

I don't think excessive quantities are likely to be the problem, though. I read a caloric breakdown once of the lifestyle of a 10th-century Scandinavian farmer; the energy requirements turn out to be absurd by modern standards, something like six thousand kcal just to stay upright at the end of the day in peak season. (Winter life was a bit more sedentary, but still strenuous by modern standards.) If you're consuming that much food regularly, an extra five hundred kcal here or there is a rounding error; it's implausible that everyone back then just happened to manage their consumption to within a few percent. Nor was the civilization as a whole calorie-bound, as best we can tell. But judging from skeletal evidence, they didn't suffer from many of the diseases of civilization that we do.

The obvious diff here is exertion, but the nutritional literature I've read tends to downplay its role. Or you could blame portion sizes relative to exertion, but larger portions are only fattening because of the excess calories, which brings us back to the original mystery. So either some novel aspect of the post-1900 diet is making modern Westerners fat, or the archaeology or the nutritional science is wrong, or I'm missing a step. And I don't think I'm missing a step.

If I had to venture a guess, I might blame lots of simple sugars in the modern diet -- honey was the only sweetener available for most of human history, and it was rare and expensive. But that's extremely tentative and feels a little glib.