From The Specialization Myth:
"The scale at which specialization of individuals leads to diversification is the city. Larger cities are more diversified than smaller cities. Among cities with similar populations – say, Salvador and Curitiba in Brazil, or Guadalajara and Monterrey in Mexico – more diversified cities are richer than less diversified cities. They tend to grow faster and become even more diversified, not only because they have a larger internal market, but also because they are more diversified in terms of what they can sell to other cities and countries."