Sexual selection is thought to be the main driver for the evolution of pre-copulatory traits (e.g., weapons and ornaments) used to gain access to mates, and post-copulatory traits (e.g., sperm form and function) used to maximize fertilisation success. Individuals that have a greater ability to acquire resources from their environment may be able to invest in both types of traits equally, and therefore may have a competitive advantage during mate acquisition and sperm competition. For males that are competitively inferior, they may choose to invest more into one type of trait, resulting in a trade-off between the investment in one trait over another. The New Zealand giraffe weevil, Lasiorhynchus barbicornis, provides an ideal study system to explore size-dependent investment in sexual traits. Males show extreme size variability, leading to differences in mating success between males of different body sizes. Large males are better at acquiring mates and winning fights and as a result, small males have adopted a sneaking tactic to avoid pre-copulatory competition with larger rivals. Therefore, small males may choose to invest more into their post-copulatory traits. Further, giraffe weevils form large aggregations on host trees during the summer months, making them an ideal subject to explore how demographic factors can influence the mating success of males. In giraffe weevils, how body size and population density interact to influence mating success is not well understood. In this study, I used an experimental behavioural assay and morphometrics to investigate how males of different body sizes may be investing into their pre- and post-copulatory traits to engage or avoid sperm competition. I found that small males may be investing more into post-copulatory traits than large males. However, I found no evidence of a phenotypic trade-off between pre- and post-copulatory traits. I also used an observational dataset collected from a natural population to explore how population density, among other demographic factors, interact with body size to influence male mating success. I found that male giraffe weevils have density-dependent mating success, with small males doing best at high densities, and large males at low densities. My research will contribute to a growing body of work looking at how sexual selection can shape the life-history traits of insects.