Reading your comment made me interested into looking into some figures to compare cities in Europe and the USA.
If you look at the population of agglomerations (some cities in Europe have ridiculously small limits - Paris is fifteen times smaller than London), you see than London (14,800,000) and Paris (11,400,000) are a bit less populated than the the biggest US cities New York (22,100,000) and Los Angeles (17,700,000) but slightly more than the third Chicago (9,750,000).
The next biggest agglomerations in Europe, Madrid (6,550,000) and Milan (6,200,000) can be compared to Houston (6,750,000), a bit less populated than San Fransisco (7,850,000).
Then you have Berlin (4,725,000) which is close to Seattle (4,425,000) or Phoenix (4,700,000).