Divergent selection in the larval stage could result in a geographic variation in adult body size. To clarify the effects of such adaptive divergence in body size on male mating behavior, spermatophore deposition and weight, copulation duration, and post-copulatory mounting were observed using male-female pairs of carnivorous Carabus japonicus beetles, showing a large geographic variation in adult body size depending on the larval environment, and the variables best predicting these behaviors were identified from individual characteristics. When the male was slightly smaller than his mate, spermatophore deposition likely succeeded, suggesting that mechanical assortative mating determined male body size. Although male reproductive organ size was positively correlated with male body size, spermatophore weight was not significantly affected by male body size, whereas copulation duration decreased with increasing male body size. Enlarged males, with a high capacity for spermatophore production, are thought to increase paternity by decreasing copulation duration and increasing mating frequency. These mating behaviors, other than spermatophore deposition, were also predicted by male and/or female populations. Thus, forms of intra- and intersexual interactions (e.g., sperm competition and sexual conflict) endemic to populations would evolve by divergences in mating behaviors associated with body size divergence.