You are right that size is one of the keys but think of this: what made Egypt and the Civilizations of Mesopotamia so populous and wealthy? The fact that they were along major rivers that allowed farmers to get silt rich water that could grow crops. China began along the Yellow and Yangtze Rivers which traverse along the breadth of China proper. Also government projects to ensure better results from farming as well as making more farm land available not only made larger populations possible but necessary to tend the land. During the Qin Dynasty Li Bing was responsible for the Dujiangyan irrigation system in Sichuan province which made it possible for about 5000 square kilometers of land to be irrigated with flooding like that of the seasonal ebb and flow of Egypt's Nile fields. Simply put, access to large amounts of farmable land means lots of food thus larger populations.