Individuals of different sex or age can vary in their prey and habitat resource use due to differences in behaviour, life history, energetic need, or size. Harbour porpoises are small cetaceans that need to feed constantly to meet their high metabolic demands. In West Greenland, the species has a unique offshore, deep-water ecology. Here, we use bone collagen carbon (delta C-13) and nitrogen (delta N-15) isotope compositions to elucidate sex and size differences in the foraging ecology of harbour porpoises from this region. Female harbour porpoises are larger than males; we find females have a higher trophic level, and delta N-15 significantly positively correlates with size for females. This indicates that size may matter in the ability of females to handle larger prey and/or dive deeper to catch higher trophic level prey. The results suggest that females, which also nurse their calves, may be under different ecological constraints than males. We also analysed the harbour porpoise data with available stable isotope data from Greenland populations of belugas and narwhals. We find that harbour porpoises have a lower trophic level than the other species, which is consistent with their smaller body size, and their diet consisting primarily of capelin. Furthermore, harbour porpoises have the largest ecological niche of the three species, in accordance with tagging studies indicating they have a wider range than belugas and narwhals and occur in shelf and deep offshore waters of the sub-arctic and North Atlantic.