Despite the wide prevalence of alternative reproductive tactics, little attention has been paid to why reproductively parasitic males are so small. In this study, we tackled this issue in a shell-brooding fish Lamprologus callipterus. Sneaky 'dwarf males' of this fish remain much smaller than bourgeois conspecifics throughout their life and employ a unique parasitic tactic, i.e. entering into a gastropod shell where a female is spawning, passing through the space between the female and shell wall and staying behind her to ejaculate throughout the spawning event. Here, we tested the prediction that they remain small to get past her through the shell spaces by interpopulation comparison. We showed, across populations, a negative allometry for sexual size dimorphism, an exponential increase of female size with an increase in shell size and a negative correlation between the magnitude of sexual size dimorphism and shell size. These results suggest that the inner spaces strongly regulate dwarf male size. We conclude that the small bodies of dwarf males arise from adaptation to their unique reproductive behaviour.