i see the opposite a lot, too, though: articles that don't need a picture but have some marginally-related stock photo tacked on just so there's a picture.
also consider the labor involved in upkeep. this is one of the rationales for nightwear: it's much easier to wash a change of nightclothes every so often than large sheets and blankets.
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.