These data are linked to the article "When Rensch meets Foster: Insular gigantism reduce sexual dimorphism in anurans". In this study, we tested whether Rensch's rule can apply to the change in body size observed in insular contexts. Here are reported the data linked to our specific comparison of continental and insular populations of a widespread coastal amphibian (Pelobates cultripes), in Western France. In this dataset, for each site, we reported the date of capture, individuals' distance to the ocean (in m), the size of individuals (SVL: Snout Vent Length, in mm), their mass (in g), their Scaled Mass Index (SMI), and their sex (note that, in some sites, some individuals were not sexed, and were thus classified neither as males or females).