In the search for a better administrative functioning as a key dimension of economic performances, changes in municipal boundaries and the creation (or suppression) of local administrative units reflect a progressive adjustment to a spatially varying population size and density. With intense population growth, municipal size reflects the overall amount (and spatial concentration) of services and infrastructures, being functionally related with agglomeration economies, land availability for building, and specific sociodemographic attributes of local communities. Based on these premises, the intrinsic relationship between settlement expansion, population growth, and municipal size in a metropolitan region of Southern Europe (Attica, hosting the Greater Athens’ area in Central Greece) was investigated in this study over nearly one century as a contribution to a refined investigation of the (changing) organization of local administrative units under a complete metropolitan cycle from urbanization to reurbanization. Based on descriptive statistics, mapping, (parametric and nonparametric) correlation coefficients, and econometric techniques, a quantitative analysis of the relationship between population size and density and municipal area provides pivotal knowledge to policy and planning adjustments toward a more balanced spatial distribution of population and administered land among local government units. Together with a slight decrease in the average municipal size over time, the average population density per municipal unit increased systematically, with a considerable reduction in spatial heterogeneity of settlements. The observed goodness-of-fit of the linear model explaining municipal area with population density, increased significantly over time. Empirical results of our study indicate that municipal size has slowly adjusted to population density across metropolitan areas, determining an imbalanced spatial distribution of resident population and a supposedly less efficient government partition. The recent administrative reform of local authorities in Greece (the so-called ‘Kallikratis’ law) seems to consolidate–rather than rebalance this organizational structure over space, reflecting spatially polarized settlements. Such conditions represent a base for informed analysis of the spatial structure of local administrative units as a pivotal element of economic sustainability and may contribute to the debate on the optimal size of municipalities at both urban and metropolitan scales of governance.