Understanding the mechanical properties of leather is crucial for expanding its range of application, but the influence of collagen fibril bundles (FBs) remains unclear. In this study, 12 kinds of bovine leather were prepared with varying FB sizes using Cr(III), Zr(IV) and aldehyde as the crosslinking agents along with different fillers such as rapeseed oil phosphate, polyacrylic acid and their combination. The experimental results revealed that the tear strength of leather was affected by the crosslinking agents, which could be further adjusted by the filler. Accordingly, a simple method using mercury intrusion porosimetry has been proposed for determining the FB size based on the crack-bridging model. Specifically, the tear strength of leather showed a strong correlation with the FB radius where the strength increased with the decrease in FB radius. This indicated that FBs served as the elementary units contributing to loading strength. These findings may facilitate the development of cleaner technologies for fabricating high-performance leather through the regulation of FB size.