Sexual size dimorphism (SSD) arises when natural selection and sexual selection act differently on males and females. Rensch’s rule predicts that SSD increases with body size in species when males are the larger sex. However, male-biased SSD is rare in insects and the rule does not always hold between species and even among populations. Here, we investigate intra-specific variation in SSD as well as relative investment in precopulatory (horn length) and postcopulatory traits (sperm length and testes weight), in a tropical rainforest dung beetle Onthophagus babirussa across Singapore and Peninsular Malaysia. Overall, three out of four populations displayed significant male-biased SSD but contrary to Rensch’s rule, SSD was greater in populations with smaller overall body size. Average male body size was similar across all populations, but female body size differed significantly suggesting that the pronounced SSD may also be due to weaker fecundity selection on female body size. Across all populations, horn length showed a strong positive static allometry while postcopulatory traits showed negative allometry (in all but one population), which suggests an evolutionary trade-off between precopulatory and postcopulatory traits in this species.