For bats, when secondary sexual dimorphism is significant, females typically are larger than males. Moreover, in mammals, variation in the degree of sexual dimorphism often follows an allometric relationship whereby differences vary with body size (Rensch's rule). We examined sexual dimorphism across the New World clade of Myotis regarding species-specific and clade-wide patterns of body, cranium, and wing size, Rensch's rule and degree to which such morphological variation is related to phylogeny. Size differences were common with significant cases of both male-biased and female-biased sexual dimorphism. In more than half the cases, females were larger than males. Variation in degree of dimorphism exhibited an allometric pattern. Nonetheless, slope of the relationship between size and degree of dimorphism was no different from unity (i.e., isometry), failing to support Rensch's rule. There was a strong and significant relationship between phylogeny and morphological variation but not between phylogeny and degree of dimorphism. Patterns suggest that differences between males and females enhance aerodynamic capabilities of females whereby larger mothers can overcome constraints on flight due to the mass of large fetuses and newborns.