Earlier quoted context omitted.
In my book, 1M people is when a city starts to become a “proper” city. It’ll host major events, invest in public transit (beyond bus), have extensive infrastructure, have extremely thorough airport service, likely have many walkable neighborhoods, and generally feel like a city. Under 1M and the city center tends to be very small with most people driving from the suburbs.
We're talking metro area, not city. If the city is 1M, it's metro area is likely 1.5 to 3 million. If the metro area is 1m then the city is probably 750k to 350k people. The post above said "The Concepción metro area is 1 million people" which means the city itself is much smaller. In fact, Wikipedia says it's 200k people, 1m metro area
Just having grown up in LA (the city), I think there's a cognitive bias in these places against recognizing just how freakin large they are when you live there. There are large corners of my home metro area I've never been to at all, whose people I would almost never interact with... and I say that as a former taxi driver! Of course, taxi driving in LA was always balkanized so you'd need separate licenses to pick up in this or that suburb... very Snow Crash-like even in the 90s.