Metro area population is a better comparison though because city political boundaries are arbitrary and vary greatly city to city and country to country. The political boundaries in the US are much older than in China, for example, so they correspond a lot less well to where population centers actually ended up being, especially post-automobile when suburbanization allowed people to easily live outside the existing boundaries of a city's limit. And then, crucially, the city's limit was then never redrawn to accurately reflect all the people actually making up a part of that city's economy. Pull up a map of China from 1930 and it's completely different from today, whereas a map of the Northeast US from 1930 is nigh indistinguishable from today.