Understanding how different behavioural and life history traits interact is fundamental to developing ethological theory. Here we study the interaction of male–male competition for mates and sexual size dimorphism in a solitary wasp, with implications for sex allocation. In Hymenoptera, females are normally larger than males suggesting that males do not benefit as much as females from larger size. However, in our focal species, a solitary Eurytoma wasp, males compete for mates by pairwise contests at female emergence sites, suggesting that male size may strongly affect fitness. In contests observed in the field, larger males were more likely to win fights, and males fighting at female emergence sites were much larger than average males. Males showed higher variance in body size than females, such that all the smallest individuals were males, a majority of medium-to-large individuals were female, but the majority of largest individuals were male. Our data suggest that sexual size dimorphism in this species has been affected by intra-sexual selection for male size, which may have implications for sex allocation.