Fecal particle size is a measurement of the results of chewing, and it provides important information on the feeding and digestion of herbivores. Understanding the effects of the proximate determinants on fecal particle size helps to interpret this widely used measurement. In folivores, previous studies found that diet composition, dietary toughness, and age-sex related factors influenced fecal particle size. However, the role of these factors remains unknown in frugivorous and omnivorous primates, which limits the interpretation of the results. This study aims to clarify how these factors influence fecal particle size in omnivorous Japanese macaques in Yakushima. We simultaneously documented Japanese macaques’ diet, dietary toughness, and fecal particle size in Yakushima. Unexpectedly, fecal particle size showed limited differences across months and no difference among age-sex classes. Dietary toughness showed no effects on fecal particle size, while the consumption of fruits showed only a marginally significant negative effect. This lack of variation might derive from a physically unchallenging diet. We found that the physical structure of preferred foods played an important role in the variations. These results suggest that food comminution is less variable in frugivorous and omnivorous primates compare to highly specialized species.