Body size has widely been recognized as one of the most important determinants of organismal form and function (1). Extremes in size can be especially illuminating of the drivers and constraints in body size evolution, and marine mammals provide a remarkable set of test cases because their independent invasions of the sea have produced a stunning array of sizes, from sea otters to blue whales that span several orders of magnitude in body mass. Why should whales have become so huge and why aren’t they larger? One scale-dependent physiological process that may shed light on these questions relates to energy flux: how animals obtain energy from the environment and how they use stored energy (2). Although larger animals have higher absolute metabolic requirements than smaller ones, the rates are not proportional to body size (3). The negative allometry of metabolic rate, whereby larger animals exhibit lower mass-specific rates of metabolism, confers a suite of physiological and ecological benefits at greater body sizes. These advantages include a low cost of transport (4); enhanced fasting ability (5); and, for air-breathers in aquatic environments, the ability to dive longer and deeper in search of food (6). There is a tendency of many vertebrate lineages to increase body size through time, but the environmental drivers and physiological constraints of body size evolution remain poorly understood. In PNAS, Gearty et al. (7) analyze the evolution of body size in several independent mammalian lineages that underwent land-to-sea transitions in their evolutionary histories. They use comparative phylogenetic analyses and an examination of the fossil record to test several hypotheses that attempt to explain why mammals evolved larger size in aquatic ecosystems.