This chapter synthesizes the theoretical and empirical insights developed throughout the book, which examines the evolving spatial dynamics of regional economies in Japan through the lens of Borrowed Size. While traditional agglomeration theory emphasizes internal scale economies within metropolitan areas, this book redefines Borrowed Size as a multidimensional concept encompassing external economies, interregional network structures, and institutional-relational foundations. Drawing on empirical chapters that employ panel data, spatial econometric modeling, and efficiency analysis, the book demonstrates that regional performance, measured by productivity and innovation, can be enhanced through access to external resources, provided that institutional connectivity and absorptive capacity are present. This chapter consolidates findings from earlier chapters and positions Borrowed Size not as a derivative of geographic proximity alone but as a dynamic capability rooted in strategic network positioning and policy coordination. Furthermore, the chapter addresses the duality of Borrowed Size and Agglomeration Shadow, arguing that regions must navigate both opportunities and risks in their interdependencies with urban centers. By integrating theory, empirical evidence, and policy implications, this chapter presents a renewed framework for understanding spatial development and proposes Borrowed Size as a foundational concept for promoting regional resilience and inclusive growth amid demographic and economic challenges.