The real question is why does it seem that 250-300 lbs people were rare in pre-industrial ages and are rather common now (yes, I don't know of good data on the prevalence of obesity before XIX century either...).

I agree that this is the real question; what surprised me was Eliezer's confident empirical statement on a subject where all the weak data I have points in the opposite direction. It looks to me like the historical data suggests that calorie availability and sedentary lifestyles might be the primary explanations (and of the two, I would expect calorie availability to have a larger impact).