<p>Although body size is linked to nearly every aspect of an organism’s biology, its evolution remains poorly understood. Here, an analytical life history model of optimal size is tested for the first time using unprecedented data on mammal life history and phylogeny and shown to explain several well-defined macroevolutionary patterns. The evolution of body size is explained in large part by life history optimization with respect to adult mortality under metabolic constraints on productivity. The model also explains plausible effects of climate change, diet, feeding mode, cursoriality, aquatic living, powered flight, and island endemicity on the evolution of body size. This study furthers our understanding of the relationship between micro- and macro-evolution.</p>