This chapter revisits the theoretical underpinnings of agglomeration economies by introducing Borrowed Size as a framework for understanding spatial dynamics beyond physical proximity. Traditional models of urban and regional development emphasize endogenous scale and geographic concentration; however, contemporary growth trajectories are increasingly shaped by interregional connectivity, functional complementarity, and institutional embeddedness. Borrowed Size denotes the capacity of smaller cities and peripheral regions to capture the benefits of agglomeration through strategic positioning within broader urban networks. The chapter synthesizes a range of theoretical advancements, including network externalities, relational proximity, and regional innovation systems, to provide a multidimensional perspective on spatial externalities. It argues that effects typically associated with the regional scale can be transmitted through nonspatial channels such as institutional trust, knowledge flows, and social networks. In doing so, the chapter establishes a conceptual foundation for rethinking agglomeration not merely as a function of size but as a relational and infrastructural condition shaped by nodal position and systemic participation.