What source did you get that from because the sources I found said over 7 million, anyway the reason the quiz did not have Hong Kong is because wikipedia excludes Hong Kong and Macau in their list
Xiamen actually has only about 3 million population, yet Jetpunk counted surrounding larger cities (Zhangzhou, Quanzhou, etc.) as part of the Xiamen metro area because Xiamen is the most developed city in that region.
Bonus points for suzhou != shanghai