Images of men in the media are becoming more muscular. The influence that the overrepresentation of idealized bodies in Western media plays on women’s body dissatisfaction has been well-documented, but less is known about how similar shifts in prevalence affect men. One idealized trait of masculinity is muscularity. In this study, we propose that prevalence-induced concept change may be one of the cognitive mechanisms underlying shifts in masculine beauty standards. We conducted an online within-subjects experiment with young cisgender men (N = 164) and found that when the prevalence of muscular bodies in the environment increased, participants increasingly judged objectively average bodies as nonmuscular. This effect was not present when men made judgments about overweightness. Computational modeling (hierarchical drift–diffusion modeling) revealed that concept change was driven by less cautious responding when judging muscular stimuli. Taken together, the current results show that men’s judgments about other male bodies and their own bodies are biased by an overrepresentation of muscularity.