This may be one of the rare usages of 'exponential' in the popular press that I can allow. The exponential aspect of urban growth may be apt -- the premise that

This may be one of the rare usages of 'exponential' in the popular press that I can allow. The exponential aspect of urban growth may be apt -- the premise that the number of people who can find a reason to relocate to a city will vary in direct proportion to the number of people who are already there might hold with acceptable accuracy over some range, and departures may be negligible during some periods of rapid growth.