There is growing global concern concerning South African learners’ mathematics achievement, with research identifying many factors contributing to poor mathematics achievement, with class size being one of them. When classrooms are overcrowded, this may lead to a stressful environment which can negatively impact the quality of teaching and learning that is possible. The purpose of this study was to gain a holistic understanding of the problem of overcrowded classrooms by using an explanatory sequential mixed-methods approach. We first investigate the association between large classes and mathematics achievement using the TIMSS 2019 data for Grade 9 mathematics. This is followed by the qualitative phase, which elicited educators’ perceptions of the relationship between overcrowded classrooms and achievement. The quantitative results showed a significant negative correlation between the two, indicating that the larger the class size, the worse the achievement. It also showed that learners taught by teachers who strongly believed that there were too many learners in a class performed significantly worse than learners taught by teachers who did not feel this way. The qualitative results showed that participants believed that overcrowded classes led to limited personalised individual interaction with learners, didactic neglect and poor time management, which are associated with poorer mathematics performance. Recommendations include that the issue of overcrowded classes should be prioritised by all stakeholders at the different levels.