This paper estimates theoretically optimal sizes of urban population for major metropolitan areas using an urban economy system with utility maximizing household, profit maximizing producer and government providing public goods. This finds that the optimal size of urban population is determined by technological levels and public services. The population sizes of Seoul, Busan, Daegu and Incheon are higher than their optimal levels, while Gwangju, Daejeon and Ulsan need to increase the population for production efficiency.