Using the OECD's Programme for International Student Assessment (PISA) study, we investigate whether students' relative ability in mathematics (in comparison to their school peers) is linked to their tendency to overclaim. Although the estimated effect size is modest (around 0.1 standard deviations) we find empirical support that being a big fish in a small pond is linked to overclaiming, with this robust to different analytic approaches and model specifications. Thus, being one of the highest academic achievers within a school may push young people's beliefs in their own abilities too far, straying into overconfidence and making claims about their knowledge and skills that they cannot justify.