part of it is just raw obesity increase, but part is also an aging population. even if women today WERE the same size as women of the same age 30 years ago, the average over the total population would still be up.

Mostly though it's the obesity increase.

40% of Americans are obese, and 75% are overweight. 30 years ago only 20% were obese.

Ah, yeah, the aging population is a good point.

I can't find a citation now, but I recall reading at some point that weight gain with age (in adulthood) didn't used to happen very much before the obesity epidemic, though nowadays we take it as a given. I wish I could find a source for/against that idea, I'm curious now if it's true.