Climate change is raising average temperatures and intensifying the frequency and severity of heatwaves, threatening animal populations. Ectotherms are especially vulnerable to increased temperature due to their limited capacity for thermoregulation, with studies indicating reduced survival under heat stress. Within-population variation in mortality under stressful temperatures could limit our ability to make broader ecological predictions. For example, males and females can exhibit physiological and behavioural differences that extend to differences in heat sensitivity. Body size can also influence temperature tolerance and could contribute to such differences. To this end, we exposed male and female burying beetles (Nicrophrous vespilloides) of a wide range of body sizes to a simulated heatwave of 26°C and examined the resulting mortality rates compared to control beetles kept at 20°C. We found that beetles exposed to the heatwave suffered significantly increased mortality, but there was no difference between males and females. We also found that a smaller body size was associated with significantly higher mortality following exposure to the simulated heatwave. This size-dependent effect suggests that warming-induced reductions in body size may also indirectly result in higher mortality rates, thereby exacerbating the effects of climate change on animal populations. Overall, our findings highlight the importance of considering sources of within-population heterogeneity when measuring responses to climate change, as traits such as body size may mediate thermal sensitivity even in the absence of sex-specific differences.