I think that is not the case. What I think is obvious is that in the eyes of artists and lay public ( other prehistoric audiences ) obese bodies were easier to portray, and attracted more viewers and awe, becausse of the same reasons it would today. Whereas a thin sexy figure immediately evokes a sexual response and arousal, the obese figurine was easier to make, and it provoked laughter, or at least, smiles.

In the times of early Cro Magon, and Neanderthal men as well, an obese woman, one with the proportions the Venus figurines represented, would become an easy meal for predators, because obviously, these obese women would be at a disadvantage in regard to hunting and running away from danger.

Same as old age, which signals the end of most animals of any species,( because of slow reflexes, loss of swiftness, of strenght, of acuteness in mind and physcial prowess ) for the obvious reasons of being less and less able to confront danger and a loss of alertness, obesity and other forms of extremes in build would render an individual prone to trouble at that right time when swiftness and alertness is mandatory, like when being surprised by a predator. That would be the reason why, w/o todays copnfrots and medical care, individuals were alive only while at their physical peak.

There was no place in the times of nomadic early Cro Magnon and Neandethal civilizations for obese individuals in the same way older individuals did not fare well.

40 was considered old, by that time, a human being and a Neanderthal man, our cousins, were already past their prime, and soon to sucumb to predators or body malfunctions.

An obese individual would had been as past from prime as an old individual.

One quibble — You say the Venus of Willendorf couldn’t stand up. She obviously couldn’t stand on a flat wooden or rock floor. But could she stand in dirt or mod? It looks to me like nice pointy spikes with a stopper at the knees.

I don’t know where they would have wanted to stand the venus, but it seems plausible at first sight that they’d have had more places available for a spike bottom than for a footed statue. And what kind of feet would it have needed to balance that wide load?