Student evaluation of teaching (SET) is important, commonplace and may be used in staff performance management. The SET literature suggests that class size is a negative systematic influence on SET ratings. In this paper we investigate time-series SET data from a large first-year engineering class where a decline in SET ratings was observed over time as course enrolment increased. We observe a negative halo effect of increasing class size on mean SET ratings and conclude that increasing course enrolment leads to a significant reduction in all mean SET ratings, even when the course learning design remains essentially unchanged. We also find an additional differential effect of increasing course enrolment on mean SET ratings. We observe that the marginal reduction in mean SET ratings for each additional student in the course enrolment is greater for those aspects of the student learning experience that are likely to be most directly impacted by increasing class size. We provide implications for practice from these findings.