TJust read the requirements for a city in the US... "Common population definitions for a city range between 1,500 and 50,000 people, with most U.S. states using a minimum between 1,500 and 5,000 inhabitants"
Wow!!! I am really surprised...
As far as I can tell, here in the US, a "city" is usually just "the local population hub". It's very relative.
So in a very rural area, the kind of place that's mostly farms and tiny villages of the "gas station and a church" variety, the biggest community in the county might have 20,000 people and get called a "city" by virtue of it being the only place around big enough to have a library, a college, and Wal-Mart.
In more densely populated parts of the US, a community of 20,000 might be counted as a small suburban town, and a place would need 100,000s of people to even count as a small city.
It just depends.