In the present study, the correlation between sperm number, sperm quality (speed, viability, longevity and length), sperm bundles quality (size and dissolving rate) and male body size has been tested in the eastern mosquitofish Gambusia holbrooki a poecilid species characterized by coercive mating tactics where males do not possess obvious ornaments, and the body size is the key determinant of pre-copulatory male mating success. The results do not tally with theoretical predictions. Indeed, no correlation between male body size and either sperm or sperm-bundle traits has been found, evidencing the lack of the theoretically expected trade-off between the investment in characters involved in mate acquisition and the investment in ejaculate quality. An explanation for the observed pattern comes from the extremely dynamic mating system of G. holbrooki, characterized by variable size-related male mating success and strong post-copulatory selective pressure, with all males facing a similar high level of sperm competition. In this situation, a higher investment in growth and maintenance at the expense of ejaculate quality is not expected. These results underscore the necessity to comprehend detailed information on species’ reproductive biology and reproductive environment to understand both the evolution of ejaculate characteristics and possible deviations from theoretical predictions.