FOREWORD : Comparing cities data is tricky! The representation use a 2011 United Nations dataset published
This dataset is an aggregation of several data providers (national censuses, estimations, regional databases) who do use different methodologies
regarding population count and defining their "urban agglomeration". You may find some more useful information on
Wikipedia websites, also on
academic article (pdf) by Forstall & al.
In addition, urban agglomerations are far from being homogeneous entities.
This dataviz has no other ambition than giving a big picture on urban agglomerations main chracteristics and footprint.