Again one quiz where we don't whether to go for city proper, metro or urban area. Anyway, maybe double check the numbers for Atlanta, Agra, Almaty, Abu Dhabi, Antalya, Aden, Aleppo, Algiers, Amman...
So you're really saying that Alvaro Obregon which is a district of Mexico City with 800,000 residents, is bigger than Atlanta with urban area population of 5,000,000?
This site mostly by QM uses this order : Proper-Metro-Urban. I googled as you suggested, the first result is Wikipedia, basically the same thing I linked, "urban area 5,000,000". If you don't mind checking the other "largest cities" quizzes on this site, "city proper" is always the smallest number while "urban area" tends to be on the larger side. You are simply getting city proper and urban area mixed up.
And none of those cities are bigger than the the ones in the quiz, thanks for the feedback tho
Atlanta