Chewing enhances digestion by promoting food particle size reduction. Results in folivores, such as cattle and horses, support that efficient particle size reduction promotes the extraction of nutrients. However, fecal particle size data in fruit-eating primates suggested they did not put much effort into chewing especially in fruit-eating seasons. It raises the question that how particle size reduction influences the digestion of foods other than leaves. The answer is crucial for estimating the effects of chewing in wild animals that consume a complicated diet. In this study, we examined the effects of particle size on digestion under in vitro conditions. We selected five typical food items from the Japanese macaques’ diet and cut them into different sizes. Then, we conducted in vitro digestibility assay and in vitro fermentation assay on these substrates to simulate enzyme and microbe digestion. We found digestibility decreased when particle size increased. However, the difference between sizes varied among food categories. Compared to leaves, the influence of size is less obvious in pulp and more obvious in seeds. The results indicate that food particle size reduction is less important for eating fruits. Without sacrificing nutrient intake, frugivores and omnivores can avoid enhancing chewing through food selection.