First, even Baartman is not close what we would call “fat”, or Venus figurine globular, by any stretch. Nomadic people would have a great deal of difficulty feeding a Mike&Molly body type, assuming it would last in an environment where they often had to walk long distances, haul all their possessions, use their muscles to produce all their necessities, etc.
I assume the Venus figurines like the Skyrim mods exaggerate and celebrate what the men found mystical and different about women - pregnancy, the ability to give birth and (for Skyrim) feed the children.
I repeat my earlier contention - that before religion found the downside to too much procreation, these figures probably date from the time when religion celebrated the heavenly experience of procreation. I’m sure back then sex ranked right up there in the awe inspiring mysteries of life such as fire, thunderstorms, and stampeding herds of giant animals… and probably the most enjoyable experience of the lot.
My first thought on seeing the Venus of Willendorf was, “She’s a hunter/gatherer???” She couldn’t have been typical of women at the time, but there must have been at least one for the carver to know what a woman that big looked like. But how could she could have gone about gathering roots and firewood every day, and moving from hunting ground to hunting ground every season? A couple of WAGs…
If the hunting was good enough, her people wouldn’t have had to move that much, maybe just twice a year. Or maybe they had discovered methods of food preservation, like smoking meat, that relieved the need to hunt regularly. The Cro-Magnons of her time apparently had enough leisure time to explore caves and develop a sophisticated artistic tradition, so for whatever reason, survival wasn’t a constant issue for them. With a plentiful supply of food and less need to do the work to obtain it, obesity would have been a natural result for those prone to it.
Or she might have had special status in her tribe as a kind of queen who had minions to do everything for her, perhaps even carry her in a litter to the next hunting ground. When I was in Nigeria, obesity was looked up to as a sign of affluence, so the venus figurines might represent power and status rather than sex or fertility (although that wouldn’t explain their nakedidity).
Or as njtt said, there might have been one or two women in the tribe whose job was to be fat and supply milk in times of need, although I wonder if they could produce enough milk to make much of a difference. I’m certainly no expert in the milk production of women.
One final thought: If the figurines were meant to be fertility talismans, why are none of them (that I know of) depicted in pregnancy? Rather than heavy with child, they’re just heavy.