Population of loggerhead turtles nesting in the Mediterranean Sea has probably evolved from the North Atlantic (NA) population, but is geographically and genetically distinct. We aggregated previously published and new unpublished data, and took two approaches to comparing these populations: an empirical one based on statistical analyses of morphological data, and a physiological one based on a Dynamic Energy Budget (DEB) model. We then analyzed causes of faster growth and maturation, but smaller size at puberty and ultimate size of the Mediterranean (MED) loggerhead turtles relative to their NA conspecifics. The empirical analysis shows that MED eggs, hatchlings, and nesting adults are consistently smaller in terms of length and mass. The physiological approach suggests physiological adaptations of the MED population to higher salinity and scarcer food availability. In particular, these adaptations include an increase in somatic maintenance needs, and a decrease in energy investment to reach and maintain sexual maturity. Our study therefore offers a mechanistic underpinning of previously observed but unexplained life-history traits, and showcases an application of DEB theory as a tool for comparative analysis of two distinct populations of the same species.