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.