The Glanville fritillary butterfly (Melitaea cinxia) has a small population (Ne ~100) on the small island of Pikku Tytärsaari (PT) in the Gulf of Finland. The population has remained completely isolated for ~100 generations, which has resulted in greatly reduced genetic variation and high genetic load (low fitness). In particular, females lay small egg clutches with low egg-hatching rate in comparison with a large reference population in the Åland Islands (ÅL). Here, we analyze to what extent egg clutch size and egg-hatching rate are influenced by male population and spermatophore size. Spermatophore size increases with male body size, is smaller after the first mating, and is smaller in the small PT population. In the ÅL population but not in the PT population, egg-hatching rate increases with spermatophore size. Egg-hatching rate of PT females is higher when mated with ÅL males than when mated with PT males (heterosis), but there is no such effect on clutch size. Clutch size of ÅL females is however reduced when mated with PT males. These results indicate that both male and female traits contribute to reduced reproductive fitness in the small isolated population.