Evolutionary perspectives on obesity have been dominated by genetic frameworks, but plastic responses are also central to its aetiology. While often considered a relatively modern phenomenon, obesity was recorded during the Palaeolithic through small statuettes of the female form (Venus figurines). Even if the phenotype was rare, these statuettes indicate that some women achieved large body sizes during the last glacial maximum, a period of nutritional stress. To explore this paradox, we develop an eco-life-course conceptual framework that integrates the effects of dietary transitions with intergenerational biological mechanisms. We assume that Palaeolithic populations exposed to glaciations had high lean mass and high dietary protein requirements. We draw on the protein leverage hypothesis, which posits that low-protein diets drive overconsumption of energy to satisfy protein needs. We review evidence for an increasing contribution of plant foods to diets as the last glacial maximum occurred, assumed to reduce dietary protein content. We consider physiological mechanisms through which maternal overweight impacts the obesity susceptibility of the offspring during pregnancy. Integrating this evidence, we suggest that the last glacial maximum decreased dietary protein content and drove protein leverage, increasing body weight in a process that amplified across generations. Through the interaction of these mechanisms with environmental change, obesity could have developed among women with susceptible genotypes, reflecting broader trade-offs between linear growth and adiposity and shifts in the population distribution of weight. Our approach may stimulate bioarchaeologists and paleoanthropologists to examine paleo-obesity in greater detail and to draw upon the tenets of human biology to interpret evidence.