Abstract Aim Island populations of small mammals are often characterized by a larger body size compared with neighbouring mainland or continental populations of the same species. A number of reasons have been put forward to explain this phenomenon. The aim of this study was to test which of these hypotheses can best explain the increase of body size in common shrews (Sorex araneus) on islands. Location The fieldwork for this study was carried out on the islands of the Inner Hebrides, Clyde Islands and the west coast of Scotland. Methods This study compared body sizes of common shrews from mainland and island sites on the west coast of Scotland, based on measurements of hind foot lengths. On 10 of the 13 islands sampled, common shrews were significantly larger than on the mainland. Body size did not vary significantly among mainland populations. We used the directional contrasts method to test the relative contributions of possible factors explaining the large body size observed in the island populations. Results We found that body size of common shrews on islands was positively related to distance from mainland, negatively related to average annual temperature, negatively related to island size, and may also be influenced by the presence or absence of pygmy shrews (Sorex minutus) on the island. Main conclusions Our results suggest a role for founder events, Bergmann's rule and K-selection in determining body size of common shrews on islands.