I agree that the Munich urban area has more than Stuttgart. I looked up the populations of Stuttgart and its main suburbs/attached towns, and it was only a million or so. I think they are including Heilbronn, Tübingen and Reutlingen, but there is a lot of countryside between those places and Stuttgart.
China should be Chongqing (31 m), Shanghai (24 m), Beijing (21 m), Chengdu (20 m) and Guangzhou (18 m). Xiamen is a rather small city with six million inhabitants.
Chengdu does appear to be more highly populated than Xiamen by any measure, according to multiple different web pages I found. (I've never been there, mind you.)
I would recommend not having both Brazil and Portugal since Porto Alegre and Porto overlap. Although I would guess most people who know Porto Alegre know Porto.
Nitpicking aside, this is a great quiz :) Goes beyond the usual range of countries and also visually pleasing
Hamburg
Munich
Cologne (without Düsseldorf)
Frankfurt
would be the correct ranking for Germany.