Sexual dimorphism, defined as a phenotypic difference between males and females of a particular species, is a common phenomenon in animals. Rensch's rule describes the pattern of sexual size dimorphism (SSD) and claims that larger species generally exhibit higher male to female body size ratios. Domesticated animals offer excellent opportunities for testing predictions of functional explanations of Rensch's theory. In this paper, predictions were tested in geese by evaluating SSD in 70 breeds of domestic geese from different geographic origins (34 from Asian origin and 36 from European origin) which were compared to their wild relatives (genus Anser, 12 species). The data indicated that the body mass of different goose breeds were consistent with the Rensch's rule, where the allometry of SSD was significantly positive. Second, despite varying selective forces, scaling of SSD with body mass did not diverge according to breed origins (Anser anser and A. cygnoides groups).