Empirical evidence suggests that Rensch's rule of allometric scaling of male and female body size, which states that body size divergence is greater across males than across females of a clade, is not universal. In fact, quantitative genetic theory indicates that the sex under historically stronger directional selection will exhibit greater interspecific variance in size. Thus, the pattern of covariance between allometry of male and female body size and sexual size dimorphism (SSD) across related clades allows a test of this causal hypothesis for macroevolutionary trends in SSD. We compiled a data set of published body size estimates from the amphibians, a class with predominantly female-biased SSD, to examine variation in allometry and SSD among clades. Our results indicate that females become the more size-variant sex across species in a family as the magnitude of SSD in that family increases. This rejects Rensch's rule and implicates selection on females as a driver of both amphibian allometry and SSD. Further, when we combine our data into a single analysis of allometry for the class, we find a significant nonlinear allometric relationship between female body size and male body size. These data suggest that allometry changes significantly as a function of size. Our results illustrate that the relationship between female size and male size varies with both the degree of sexual dimorphism and the body size of a clade.