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, 2006f) 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 23). Medium-sized cities are half the economic size (GDP) of large ones and less than one-third of those with more than 1.5 million people.