Believe me, few if any people got that fat in hunter-gatherer or early agricultural societies. But women got fatter - and more fertile - in agricultural societies than in hunter-gatherer ones. Their diets weren't as healthy overall, but with a bit more body fat, they had higher birthrates.

People way back when obviously noticed that more body fat = more babies (to a point, anyway). So they associated fat with fertility. Hence the "Venus" sculptures: creating a representation of an extremely fat woman was, somehow, meant to encourage the universe or the spirits or whoever to grant them extreme fertility. Bountiful harvests, lots of babies, all that stuff.

Also, keep in mind that we don't know what the Venus sculptures were actually for. They may have been idols, they may have been one-use talismans to bring on pregnancy or a good harvest. The point is that they were probably a sort of ritual exercise in wishful thinking. "Hey god/gods/whatever! We want to have enough food to be this fat and have lots of babies!"

Good speculation. I think in general, in a fat society where people have to work hard to be thin- thinness is adored. In a subsistence society where everyone yearns to have enough to eat, fatness is adored as a sign of abundance.