Male sex-biased parasitism (SBP) occurs across a range of mammalian taxa and two contrasting sets of hypotheses have been suggested for its establishment. The first invokes body size per se and suggests that larger individuals are either a larger target for parasites, trade off growth at the expense of immunity or cope better with parasitism than smaller individuals. The second suggests a sex-specific handicap whereby males have reduced immunocompetence compared to females due to the immunodepressive effects of testosterone. The current study investigated whether sex-biased parasitism is driven by host ‘body size’ or ‘sex’ using a rodent–tick (Apodemus sylvaticus–Ixodes ricinus) system. Moreover, the presence or absence of large mammals at study sites were used to control the presence of immature ticks infesting wood mice, allowing the impacts of parasitism on host body mass and female reproduction to be assessed. As expected, male mice had greater tick loads than females and analyses suggested this sex-bias was driven by body mass as opposed to sex. It is therefore likely that larger individuals are a larger target for parasites, trade off growth at the expense of immunity or adapt behavioural responses to parasitism based on their body size. Parasite load had no effect on host body mass or female reproductive output suggesting individuals may alter behaviour or life history strategies to compensate for costs incurred through parasitism. Overall, this study lends support to the ‘body size’ hypothesis for the formation of sex-biased parasitism.