In this research we propose an ironic theory of urban concentration, city size, and democratic reform. Authoritarian regimes tend to concentrate their countries' populations in primate cities early in development. While this allows autocratic regimes to easily monitor and suppress most of their countries' would-be political entrepreneurs and incipient social movement organizations, over time such primate cities can grow very large, concentrating grievance (in the form of conspicuous inequalities and ascribed identities) in places that may also offer unique mobilization resources (e.g., international media). We propose that large primary cities can become democratic "pressure cookers" for authoritarian regimes, suggesting that a country's spaceeconomy may be an integral part of the puzzle of democratic transition. Using information from over eighty developing countries, our fixed-effects pooled timeseries analyses of city growth, antistate demonstrations, and democratization generally bear out our theory. We conclude that authoritarian rulers do enjoy political and economic advantages by encouraging or dictating urban concentration, but also that these advantages probably diminish and perhaps reverse over time. As primary cities swell in population, they form global "theaters" that amplify contention and encourage other social forces that hasten democratic reform, which can hoist authoritarianism "on its own petard." We offer this as a somewhat ironic addendum to current macrosocial theories of political contention and change.