<p>The bony labyrinth of the petrosal bone is a fluid filled organ involved in vertebrate hearing and balance. The volume of the bony labyrinth differs across mammals but generally scales with body size. However, the relative sizes of anatomically distinct parts of the inner ear also differ across the diversity of mammals. In cetaceans, for instance, the shape of the cochlea is different between those that echolocate and those that do not, and the semicircular canals vary greatly and are always much smaller than in other mammals. An understanding of the relative size of each part in terms of allometry and modern phylogeny is important for the interpretation of function. In this work, we present a broad survey of the volumes of the different parts of the therian inner ear, and use such volume measurements to address the differences in the vestibular system of fossil Eocene cetaceans. We find multiple instances of size reduction in the R_m and the relative sizes of the canals through the course of cetacean evolution. However, the earliest size reduction (in semiaquatic Eocene species) appears to be accompanied by an increase in the cross-sectional area of the canal, which has important functional implications. We further find that Eocene cetaceans show the same pattern in the relative size of the cochlear scalae as is seen in modern marine species; which is possibly an adaptation for bone conduction hearing. Our data informs our understanding of inner ear morphology in therian mammals and generates some new insights into changes in the vestibular system through cetacean evolution.</p>