Giacomo Ponzetto emails me:
I’m glad you found our recent working paper on urban networks interesting and cited it on Marginal Revolution. I’m also glad that your readers pointed out our lack of clarity concerning Busan.
As they have already noted, what we meant is that the metropolitan city of Busan would be the second largest city in the European Union by population within administrative city limits, after Greater London (8.5 million) and pretty much tied with the city-state of Berlin at 3.5 million (the latest official figures we could find are 3,563,578 for Busan in 2013 and 3,562,166 for Berlin on 31 December 2014). We find this an interesting fact. It reflects political and administrative decisions to facilitate urban integration in Asia and conversely to preserve local identities in Europe. On the other hand, we agree that administrative boundaries often don’t provide the most useful definition of a city. The problem is that no other definition is unambiguous and plainly comparable across countries.
In its 2006 Territorial Review Competitive Cities in the Global Economy, the OECD defined the Busan metro area to include the administrative units of Busan, Ulsan, and Gyeonsangnam-do. By this definition, greater Busan has around 8 million residents, which is also the figure reported by Wikipedia, without defining the metro area. As the OECD noted, this is probably an overstatement because Gyeongsangnam-do is a large province including non-urban districts. Yet, a much smaller Busan-Ulsan-Changwon metro area (including Gimhae, Yangsan, Miryang and Geoje) has around 7 million residents. It would be the third largest in the European Union, behind London and Paris (12 million residents in the functional urban area as defined by Eurostat) but ahead of Madrid (6.5 million).
However, if instead one takes the view that Busan, Ulsan and Changwon are three continguous metro areas instead of a single one, then Busan-Gimhae-Yangan would have only around 4.5 million residents. It would be smaller than Madrid, the Ruhr, Berlin and Barcelona, but at the top of the pack that includes Rome, Milan and Athens. We chose to rely on the administrative-city figures to avoid having to adjudicate which is the most accurate functional definition of Busan’s metro area. Either way, you’re right it’s all too easy to underestimate Busan.