Compared to other animal groups, mammals invest substantial amounts of energy in their offspring via gestation and lactation. Despite this unifying characteristic, strategies of maternal investment – i.e., reproductive energetics – differ considerably across species. Indicators of maternal investment strategies include gestation time, fetal and offspring growth rates, birth mass, litter size, lactation time, and weaning mass. While these indicators scale to some degree with maternal body size, comparisons among groups indicate investment strategies may differ between terrestrial and aquatic mammals. In this study, we synthesized multiple datasets from the scientific literature of approximately 3000 placental mammals, with the specific aim of comparing maternal investment indicators between marine and terrestrial groups. Our results show that (1) the effects of maternal body size on investment indicators differ between aquatic and terrestrial mammals such that (2) investment indicators tend to be higher in marine mammals than terrestrial mammals of similar size. For marine mammals these include faster fetal and offspring growth rates, as well as larger birth and weaning masses, despite shorter gestation and lactation times. These results indicate that marine mammals invest a larger amount of energy in their offspring, and more quickly, compared to their terrestrial counterparts. We posit these differences to be driven by the very different characteristics of their respective environments, whereby the greater environmental productivity of the marine environment promotes large investments of energy in offspring while, simultaneously, the sheer scale of the spatiotemporal dynamics of seasonal migrations between feeding and breeding grounds constrains aquatic mammals to do so quickly.