The effect of intraspecific contact (Contact) on egg production was examined in the massive coral Goniastrea aspera in Okinawa, subtropical Japan. The contact was non-aggressive without damaging soft tissues each other. Within Contact colonies, polyp volume, polyp fertility (%polyps with gonad), and NE/PV (number of eggs per polyp volume) were significantly smaller in marginal (Mg) polyps without direct intraspecific contact than other polyps, but no difference was found between non-marginal and Mg-Contact (marginal with direct intraspecific contact) polyps. Comparisons of non-marginal polyps (non-marginal and Mg-Contact polyps were combined in Contact colonies) between Non-Contact and Contact colonies showed that fertility and NE/PV were significantly larger in Contact colonies than in Non-Contact colonies, but polyp volume were not different significantly. Further analyses dividing colonies at Non-Contact maturation colony size (60 polyps) revealed that fertility and NE/PV were significantly larger in Contact colonies than in Non-Contact colonies only in the small colonies (<60 polyps), indicating that the intraspecific contact promoted sexual maturation at smaller colony size; one polyped Contact coral was also reproductive. The lack of correlation between polyp volume and NE/PV in the small Contact colonies, and the similarity of NE/PV in non-marginal and Mg-Contact polyps within a colony, suggest that the maturation at smaller size in Contact colonies is realized by reproductive integration of polyps at the colony level. The present results show that size-structured populations such as colonial corals may show phenotypic diversity in key demographic parameters, such as reproductive output, dependent on ecological conditions.