While I get your point, the population of Sweden just past 10 million, and Sydney, Australia's largest city is just shy of 5 million. I don't entirely agree with your point though.. If an entire country moves from one payment system to another, it's a larger deal compared to a very large city since a country is a more complete economic system than a city. And if one country makes it work, then the method will be easier to emulate in another. A city on the other hand is not governed in a nearly as complicated fashion as a country so that's not as big of a deal no matter how large a city.