Largest city is hard to define, but I think metro area population is the best measure and according to Wikipedia these are the top ten:
1 Tokyo Japan 32,450,000 8,014 4,049
2 Seoul South Korea 20,550,000 5,076 4,048
3 Mexico City[2] Mexico 20,450,000 7,346 2,784
4 New York City[3] United States 19,750,000 17,884 1,104
5 Mumbai India 19,200,000 2,350 8,170
6 Jakarta Indonesia 18,900,000 5,100 3,706
7 Sao Paulo Brazil 18,850,000 8,479 2,223
8 Delhi India 18,600,000 3,182 5,845
9 Osaka-Kobe-Kyoto Japan 17,375,000 6,930 2,507
10 Shanghai China 16,650,000 5,177 3,216
Surprising to me was that Shanghai and Hong Kong are both larger than Beijing which was in 16th place, and all three are smaller than New York City. (Where do all our billion+ Chinese friends live, anyway?)
Hey, I’ll be in two of the top ten over the next week! Hong Kong and Shanghai. Cool.
Joe Duck 
Seems to me that when I was in Shanghai, there was a daily transient population of 7 million people.
When it comes to mass amounts of people, in China a new sense of scale is necessary.