Finding accurate figures for metropolitan area sizes was a frustrating and daunting task for a lot of countries, especially non-Western countries. Some of the labeled cities on this map were difficult to pinpoint as well. For example, all sources I found listed Warsaw, Dubai and Santa Cruz de la Sierra as the largest cities in their respective countries — but I chose metropolitan areas as a more accurate indicator of an urban area’s economic importance, and these three were edged out in that race by Katowice, Abu Dhabi and La Paz.
Four European “mega-cities” also frustrated my attempts to determine a country’s second city. Milan in the north of Italy has numbers listing its conurbation population as anywhere from 4 million to 12 million. Even wikipedia can’t agree with itself. It was also difficult to decide if Rotterdam-The Hague and Amsterdam were in the same metropolitan area or two separate entities; I decided to separate them. Katowice (mentioned above) is the largest city of the Silesian Metropolis which just surpasses Warsaw. Finally, I used the metropolitan areas for the Rhine/Ruhr area in Germany which far surpasses both Berlin and Hamburg in population. However, determining population of European (especially Schengen area) megacities was complicated again by the open borders of the European Union. How much of Geneva’s metro population lives in France? Does a person in Rybnik have closer ties to Katowice or Ostrava? Finding accurate numbers for these metro areas when two different governments are responsible for the census is exceedingly complex.
One interesting thing I learned while making this map is that all three of the second-largest cities of Uzbekistan, Kyrgyzstan and Tajikistan are clustered in the Fergana Valley, an area of unusually complicated national borders, distant from the capitals of those three countries.