Apparently, during episode 8, Dixing's population is confirmed to be 900k-ish. (scene)

What really eliminates Dragon City from the 500k max category for me is the scenery we see from the University roof. The centrum-density housing goes on seemingly forever, and we know that there's a river in the city, which we have no indications of in that city background. Add to that the severely variegated nature of the boroughs and it doesn't match any of the cities I've lived in.