Although the relevance of particle size reduction in herbivore digestion is widely appreciated, few studies have investigated digesta particle size across species in relation to body mass or digestive strategy. We investigated faecal particle size, which reflects the size of ingesta particles after both mastication and specialized processes such as rumination. Particle size was measured by wet sieving samples from more than 700 captive individuals representing 193 mammalian species. Using phylogenetic generalized least squares, faecal particle size scaled to body mass with an exponent of 0.22 (95% confidence interval: 0.16–0.28). In comparisons among different digestive strategies, we found that (1) equids had smaller faecal particles than other hindgut fermenters, (2) non-ruminant foregut fermenters and hindgut fermenters had similar-sized faecal particles (not significantly different), and (3) ruminants had finer faecal particles than non-ruminants. These results confirm that the relationship between chewing efficiency and body mass is modified by morphological adaptations in dental design and physiological adaptations to chewing, such as rumination. This allometric relationship should be considered when investigating the effect of body size on digestive physiology, and digestion studies should include a measure of faecal particle size.