Estimated population sizes for prefecture-level cities in millions for which data about the resident population is available for 2014 and Guangzhou, the largest city in the Pearl River Delta.

The Gross Domestic Product is used as the estimator, see text. The corresponding values are plotted in Fig 6a. It is clear that the true population size of large coastal cities is not well described by the Hukou system count, but better captured by the estimates resulting from the residuals of the scaling relations for GDP.