[Population] dispersion is at least as important in the expansion of megacities as population growth itself. Population densities generally decline as urban areas add residents.
This is not counter-intuitive. We think city growth as a process where more people squeeze into city limits, with perhaps some limited physical expansion on the margins. In reality, city growth will be accompanied with rapid expansion of transport infrastructure, which enables commutes of a greater distance. That sets the stage for rapid physical expansion. Long-time residents move from the core to the suburbs in search for a better quality of life, or pushed out by gentrification and higher rents in the core. Some suburbs will develop their own industry clusters, which will attract yet more people. These centrifugal forces will rapidly increase geographical city size and reduce density.
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