If we're going to use arbitrary points in time, at its peak Ancient Rome had over 1M inhabitants, and Chang'an had almost 2M, all before the IX century.
A more apt comparison might be London right before the car became common place, though.
The moment in time you pick for London does not change the picture. Victorian London still didn't have a million souls.
Rome's 1M population is a disputed number. Cities of tens of million inhabitants are all post-industrial and none work solely on mass transit. The car is an essential piece of the equation.
Car-less cities are an interesting thought experiment, but none has gone past the implementation barrier. That part of the equation should go into the though experiment.
> Victorian London still didn't have a million souls.
Can you provide a link? I'm either confused about what "Victorian" or "London" mean, because it seems to me that metro London definitely had over 1M people in the late 19th century.
What city did actually function before the invention of individual motorized traffic? Rome? London? Would you want to live there back then as a simple citizen?
Is this a serious question? Let's take 1900 as an example year that's before individual motorized traffic (the car existed but clearly wasn't popularized). New York had a population of 3.5MM. London was 6.5MM. Paris 2.7MM.
If we're going to identify something key to the growth of cities, surely it is mass transit. Much of this isn't apparent in modern cities in the USA because so many of them tore apart their streetcar systems in the middle of the 20th century as policies shifted to favor the private automobile. But mass transit seems to me like the thing that drove the growth.
Well it wasn't full of diesel and petrol fumes. The great stink was forty years earlier, they already had decent sewage systems (many of which are still in place now).