House mice from 24 Scottish and Faroese islands show positive correlation of skeletal size with island area, conforming to a model of resource limitation in very small islands. Molar size is not correlated with island size, suggesting greater genetic canalization; smaller individuals have larger tooth to body size ratio. The size variation may have a simple genetic basis or may be ecophenotypic. The offset between skeletal and molar size has potential use in the fossil record as a marker for these rapid effects, while longer-term evolutionary change reverts to approximate tooth—body size isometry. Collation of related studies indicates frequently deterministic relationships of small-mammal body size to island size. The parameters of the relationship (positive, negative or parabolic) vary widely with case study according to biotic and abiotic factors. In the present study there was no relationship of mouse size to any variable (e.g. presence of competitors) except island area.