1. Body size differences among coexisting related species are common, but the actual effect of these differences in mitigating interspecific interactions, such as resource competition and reproductive interference, is poorly understood. 2. Local assemblages of the ground beetle genus Carabus (subgenus Ohomopterus) typically consist of two or more species of varying sizes. Through foraging and mating experiments using four Ohomopterus species in parapatry and sympatry, we examined whether interspecific body size differences are effective in partitioning food resources or reducing reproductive interference. 3. Because larval Ohomopterus feed exclusively on earthworms, body size differences may be related to partitioning earthworms of different sizes. However, larvae did not exhibit differences in selectivity or attack success on earthworms of different sizes based on larval body size, indicating little possibility of partitioning food by body size. 4. In contrast, interspecific mating behaviours, such as mate recognition, mounting, and copulation, were hindered when body size differences were large; copulation was frequently accomplished between parapatric species with smaller body size differences. 5. These results suggest that body size differences between species effectively reduce reproductive interference, rather than resource competition. Although body size differences in coexisting closely related species have been considered to function in resource partitioning, they may function primarily in reproductive isolation and thereby facilitate coexistence of species.