Socioeconomic status is one of the strongest predictors of obesity, and of living in deprived neighbourhoods with unhealthy food environments. Little is known, however, about the psychological processes that translate features of such environments into socioeconomic differences in eating behaviour. One important feature of unhealthy food environments is the prevalence of oversized portions of unhealthy food. The present study tested whether individuals with lower socioeconomic status intend to consume more from large portions than those with higher socioeconomic status, and examined the psychological processes underlying this effect. A large-scale online experiment was conducted in which participants (N = 511) indicated how much they would eat from small and large portions of healthy and unhealthy snacks. The mediating effects of trait impulsivity and perceptions of how much was considered appropriate to eat were also assessed. Participants with lower socioeconomic status intended to eat more from the large portions than from the small portions of the unhealthy snacks, which would equate to a potential 15-22% increase in energy intake. These effects were partially mediated by trait impulsivity and perceptions of how much is appropriate to eat. These findings point to a significant health burden of low socioeconomic status: when exposed to unhealthy food environments, specific psychological processes might increase the amount of unhealthy food those with lower socioeconomic intend to consume. This study critically informs the emerging understanding of the psychology of socioeconomic status and eating behaviour.