  Recent advances in the urban science make broad use of the notion of scaling.
We focus here on the important scaling relationship between the gross
metropolitan product (GMP) of a city and its population (pop). It has been
demonstrated that GMP $\propto$ Y pop $^{\beta}$ with $\beta$ always greater
than 1 and close to 1.2. This fundamental finding highlights a universal rule
that holds across countries and cultures and might explain the very nature of
cities. However, in an increasingly connected world, the hypothesis that the
economy of a city solely depends on its population might be questionable. Using
data for 248 cities in the European Union between 2005 and 2010, we found a
double GMP/pop scaling regime. For West EU cities, $\beta$ = 1 over the whole
the period, while for post-communist cities $\beta >$1 and increases from
$\sim$1.2 to $\sim$1.4. The evolution of the scaling exponent describes the
convergence of post-communist European cities to open and liberal economies. We
propose a simple model of economic convergence in which, under stable political
conditions, a linear GMP/pop scaling is expected for all cities. The results
suggest that the GMP/pop super-linear scaling represents a phase of economic
growth rather than a steady, universal urban feature. The results also suggest
that relationships between cities are embedded in their political and economic
context and cannot be neglected in explanations of cities, urbanization and
urban economics.
