Overgrazing by large mammalian herbivores has led to significant adverse impacts on ecosystems globally. Insects are often a key taxon affected by large herbivores because the plants that are grazed provide crucial food and habitat. By changing vegetation, overgrazing by herbivores could affect aspects of insect morphology, including through changes to larval development due to reduced food availability, and adult dispersal ability due to habitat fragmentation. We investigated the wing morphology of moth species in two contrasting sites at Lake Toya in Hokkaido, Japan. We compared moths on Nakajima Island where deer are overabundant, with moths from the lakeshore 3 km away where deer are far less abundant. We compared forewing size and aspect ratio (length/width) of 13 moth species from both lakeshore and island sites. Four species, three of which were herb-feeding generalists, had significantly smaller wings on the island compared with the lakeshore. Seven species demonstrated a reduction in wing aspect ratio, whereas one species, the largest we measured, showed an increase in wing aspect ratio. We suggest that these morphological changes could be induced by overgrazing by deer (i.e., a reduction in moth host plant biomass and quality) and/or the isolation of moth populations on Nakajima Island. Further work is needed to reveal how these confounded but potentially interacting effects contribute to the morphological changes we found in the moths on the island. Our results show that habitat isolation and overabundance of deer populations can affect moth wing morphology, with potential implications for their population dynamics and community structure.