https://phys.org/news/2018-07-city-size-crucial-role-migration.html

Inhabitants of smaller cities—of fewer than 100,000 inhabitants—are twice as likely to migrate than those of cities with a population greater than ten million. Those who do migrate are more likely to move to a city of a similar size.

People already living in larger cities are less likely to migrate, but when they do, they are more likely to move to similarly sized large cities.