The temperature-size rule predicts that for ectotherms, a rise in temperature induces a decrease in body size and an earlier age at maturity. However, this response is not universal and variations such as increased body size with earlier maturation or decreased body size with delayed maturation are also observed. Yet, most research on temperature-size responses does not account for the fact that global-change increases in mean temperatures are concomitant with changes in temperature variance. Furthermore, the temperature-induced changes on the nutritional environment of ectotherms are also rarely considered. Here, we develop a bioenergetic model of ectotherm body size and age at maturity as a function of temperature, food quantity and quality. The model is able to reproduce the entire spectrum of observed temperature-size and age responses. Simulations indicate that temperature variance shifts maturity towards smaller sizes while age is only slightly affected. When food quality becomes limiting, the effects on age at maturity strongly increase in particular at the warmer and colder boundaries of the ectotherm’s temperature breadth. We experimentally confirm this modulation of the temperature variance effects on size and age at maturity by food quality using the model organism Daphnia magna. Overall, our results indicate that accounting for the environmental changes that accompany the rise in mean temperature may yield different predictions on the amplitude and direction of the climate change effects on ectotherm body size-at-age than by considering mean temperature alone.