Theory predicts that sperm competition will favour the production of larger ejaculates. However, because the benefits of greater reproductive investment are balanced by the costs of spermatogenesis, expenditure should depend on male physiology, mating rate and the relationship between additional investment and fertilization gains. In the yellow dung fly, Scathophagastercoraria, males adopt size-dependent alternative mating tactics that are associated with discrete ecological resources (foraging and oviposition substrates), although males switch between these environments throughout their lives. By copulating on foraging substrate (fruit or flowers), males can bypass intense mate competition that occurs at oviposition sites (cow dung), but as a consequence, must occupy a disfavoured mating role (i.e. face a greater risk that their ejaculate will be displaced from storage prior to fertilization). Small males often mate on foraging substrate, whereas large males mate in this environment only during feeding bouts. Optimal ejaculate expenditure should therefore depend on male size and their current mating role. By measuring copula duration (i.e. ejaculate expenditure) of natural matings and assigning paternity to resulting offspring, we confirmed that copulations on dung sire approximately three times as many offspring as those on foraging substrate. Furthermore, large males reduced copula duration on fruit, as predicted, since this strategy enables greater investment into high-payoff matings on dung. Conversely, small males copulated for shorter durations on dung than on foraging substrate, perhaps to minimize the risk of being displaced from copula by a rival. These patterns of ejaculate expenditure translated into greater offspring production for large males on dung and for small males on fruit. We discuss the possible proximate factors driving these size- and context-dependent patterns of ejaculate allocation by yellow dung fly males. Together, our findings shed light on the allocation strategies and reproductive consequences of alternative mating tactics.