Many studies have demonstrated the adaptive advantage of elaborate secondary sexual traits, but few if any have shown compelling evidence for the limits to the elaboration of these traits that must exist. We describe such evidence in the exaggerated mandibles of stag beetles. In 1932, Huxley showed that the slope of the allometric relationship between mandible length and body size in some stag beetles declines in the largest males. We show that this curvature is most pronounced in species with relatively long mandibles, consistent with the hypothesis that the decrease in slope is caused by the increasing costs of large mandibles, which ultimately limit their size. Increasing depletion of resources in the prepupa and pupa by the rapidly growing mandibles is the most likely way in which these costs are manifested. The curved allometries have two components: intraspecific mandible allometry is steepest among small males of the species with the longest mandibles, but shallowest among the largest males of those same species. These patterns suggest that selection continues to favour positive allometry in species that invest relatively more in weaponry despite the limits to mandible exaggeration being reached in the largest males.