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. . . .
[I]nternational migration follows a different pattern, with individuals more likely to head to a large city where they are perhaps more likely to find jobs, housing and other people of a similar culture.From here. The threshold for city status in the study was 50,000 inhabitants.