The article by Wolf U. Blanckenhorn explores the mechanisms that limit the evolution of larger body sizes in organisms. While fecundity selection and sexual selection are widely recognized as major forces driving larger body sizes, the counterbalancing forces that keep organisms small are less well understood. The author reviews various ecological, evolutionary, and physiological mechanisms that could limit body size, including viability costs in juveniles and adults, reduced mating success due to increased visibility and energy requirements, and the energetic demands of reproduction. Despite extensive theoretical and comparative evidence, empirical studies demonstrating these costs are scarce. Blanckenhorn emphasizes the need for experimental studies in multiple environments to better understand the fitness consequences of large body sizes. He also highlights the importance of integrating data from different fitness components and using life-history simulations to address the complex interactions involved. The article concludes by calling for more comprehensive case studies and genetic research to elucidate the mechanisms that keep organisms small.The article by Wolf U. Blanckenhorn explores the mechanisms that limit the evolution of larger body sizes in organisms. While fecundity selection and sexual selection are widely recognized as major forces driving larger body sizes, the counterbalancing forces that keep organisms small are less well understood. The author reviews various ecological, evolutionary, and physiological mechanisms that could limit body size, including viability costs in juveniles and adults, reduced mating success due to increased visibility and energy requirements, and the energetic demands of reproduction. Despite extensive theoretical and comparative evidence, empirical studies demonstrating these costs are scarce. Blanckenhorn emphasizes the need for experimental studies in multiple environments to better understand the fitness consequences of large body sizes. He also highlights the importance of integrating data from different fitness components and using life-history simulations to address the complex interactions involved. The article concludes by calling for more comprehensive case studies and genetic research to elucidate the mechanisms that keep organisms small.