Several species are characterised by male mating polymorphisms, which are often associated with sperm competition for which some phenotypes show specialisation. With high-resolution behavioural data, we used network analysis to quantify sperm competition intensity; the probability of overlap of the ejaculates of different males among competing male European bitterling (Rhodeus amarus) in a large experimental mesocosm. Implementing Bayesian inference with informative priors, we modelled sperm competition intensity among males adopting two alternative mating roles to understand how sperm competition intensity varied between roles as a function of body size. We demonstrated that larger males performing a “guarder” role experienced lower sperm competition intensity than smaller males. In contrast, for “sneaker males”, larger males experienced greater sperm competition intensity. We discuss these findings in the context of mating system evolution and the reproductive biology of bitterling.