1. For their larval development, parasitoids depend on the quality and quantity of resources provided by a single host. Therefore, a close relationship is predicted between the size of the host at parasitism and the size of the emerging adult wasp. This relationship is less clear for koinobiont than for idiobiont parasitoids. 2. As size differentiation in host species exhibiting sexual size dimorphism (SSD) is likely to occur already during larval development, in koinobiont larval endoparasitoids the size of the emerging adult may also be constrained based on the sex of the host caterpillar. 3. Sex-specific growth trajectories were compared in unparasitised Plutella xylostella caterpillars and in second and fourth instar hosts that were parasitised by the solitary larval koinobiont endoparasitoid Diadegma semiclausum. Both species exhibit SSD, where females are significantly larger than males. 4. Healthy female P. xylostella caterpillars developed significantly faster than their male conspecifics. Host regulation induced by D. semiclausum parasitism depended on the instar attacked. Parasitism in second-instar caterpillars reduced growth compared to healthy unparasitised caterpillars, whereas parasitism in fourth-instar caterpillars arrested development. The reduction in growth was most pronounced in hosts producing male D. semiclausum. 5. Parasitism itself had the largest impact on host growth. SSD in the parasitoid is mainly the result of differences in growth rate of the parasitoid–host complex producing male and female wasps and differences in exploitation of the host resources. Female wasps converted host biomass more efficiently into adult biomass than males.