Engineering small-molecule drugs into nanoparticulate formulations provides an unprecedented opportunity to improve the performance of traditional chemo drugs, but suffers from poor compatibility between drugs and nanocarriers. Stimuli-responsive mPEG-PDLLA-drug conjugate-based nanomedicines can facilitate the exploitation of beneficial properties of the carrier and enable the practical fabrication of highly efficacious self-assembled nanomedicines. However, the influence of hydrophobic length on the performance of this type of nanomedicine is little known. Here we synthesized two acid-sensitive ketal-linked mPEG-PDLLA-docetaxel prodrugs with different lengths of PDLLA, and engineered them into self-assembled sub-20 nm micellar nanomedicines for breast cancer chemotherapy. We found that the nanomedicine consisting of a mPEG-PDLLA-docetaxel prodrug with the shorter length of PDLLA stood out due to its potent cytotoxicity, deep penetration into multicellular spheroids, and improved in vivo anticancer performance. Additionally, our prodrug-based nanomedicines outperformed the generic formulation of commercial Nanoxel in terms of safety profile, tolerated doses, and tumor suppression. Our findings indicate that the hydrophobic content of a polymeric prodrug nanomedicine plays an important role in the performance of the nanomedicine, and should be instructive for developing polymeric prodrug-based nanomedicines with clinical translational potential. & COPY; 2023 Published by Elsevier B.V. on behalf of Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences.