it seems to have to do with the fact that everyone has different visual signifiers as to what constitutes a "big city."
a scene like this contains the idea of "big city" to me, in a way, but geneva is not a big city.
(credit: columbusmmug.com)
because i am seemingly wired to seek out these sorts of scenes in order to feel like i am approaching a sizeable city, i would probably tend to underestimate a place like edmonton. similarly, if one looked more to things like skylines, a place like vienna, and a scene like this within vienna, might not be enough to convince a guy from calgary or denver that he is indeed within a big city.
it gets subjective and aesthetic, even where the numbers are readily available.