nevertheless given its economic structure it may be useful to think of a particular city having an optimum size the oecd report competitive cities in the global economy oecd f shows that the association between income and city size turns negative once a threshold is reached and diseconomies of scale become more important than agglomeration economies in general very small and very large cities tend to have lower economic growth rates than the average urban area in the oecd figure mediumsized cities are half the economic size gdp of large ones and less than onethird of those with more than million people