Here is a table of various developed countries, as well as some regions of the US, based on how dominated they are by their largest respective cities. The table includes the percentage of the population in the top metro area, and the ratio of the top metro area’s population to that of metro areas 2-4. Different countries have data from different years, but within each row, the data is from the same year. Of course the definitions of metro areas are not consistent from country to country, but I’ve tried to use the more expansive definitions where they are available.
* Sacramento excludes the one county it has in Nevada.
** The Northeast includes Washington, the states to its north and east, and its suburbs in Virginia and West Virginia.
I could not find data for Switzerland, but the Canton of Zurich is 17% of national population; including the neighboring cantons of Schaffhausen, Zug, and Schwyz (all with Zurich S-Bahn service) raises this to 22%; including also Aargau, also with Zurich S-Bahn service, raises this to 29%.
The takeaway from this table is that our usual notions of which regions are more capital-centric (France, UK, Japan, South Korea) and which are less (Germany, most regions of the US) are more than just about the capital’s share of the population. Germany’s difference with France is not just the largest metro area’s share of the population; it’s also the difference between political centralization around Paris and the polycentric economy of Germany, and this is seen in the second-city shares. (And on top of this, the Rhine-Ruhr region is itself highly polycentric, and the stricter definitions of metro area break it into three.)
In the Northeastern US and California, we see huge largest-cities, but also strong second cities. This is true both demographically and politically, and this is why we can expect travel between Los Angeles and San Francisco or between New York and Washington to be much more symmetric than in France or the UK.