ABSTRACT Early life conditions can have long‐lasting effects on fitness. In insects, larval crowding increases intraspecific competition, shaping adult phenotypes and influencing male reproductive success in pre‐ and post‐mating competition. Although some larval crowding effects on seminal fluid protein (Sfp) allocation are known, studies often focus on a small subset of Sfps and overlook male–female interactions. A comprehensive understanding of how male and female larval environments interact to influence seminal proteome composition and transfer is still lacking. Here, we manipulated Drosophila melanogaster larval crowding (low vs. high) to generate large and small adults and mated individuals in a fully factorial design. We then measured Sfp production, composition and transfer. Large males produced relatively higher quantities of Sfps. However, small males transferred greater quantities of a subset of Sfps. When examining proteins individually, 10 Sfps were transferred at significantly higher abundances by small males than large males. Our findings suggest that small males invest more per mating, potentially due to fewer mating opportunities or cues of high larval density influencing reproductive strategies. This study provides new insights into early life effects on ejaculate allocation in D. melanogaster , highlighting physiological and behavioural responses to developmental conditions. Understanding these mechanisms offers valuable perspectives on reproductive strategies and fitness trade‐offs in insects.