Very good point on more people. In 20 years, US population went from approx 280,000,000 to 330,000,000, about 18% more people.
https://www.worldometers.info/world-population/us-population/
Good thing in the lower graph on that page is that the rate of growth is decreasing. Check out Europe's population on the same link and see why they're almost encouraging immigration - not enough people to support a country at some point.
Tons of factors in all this and I pick the ones that match my ideology - not always wise when I try to claim some sense of objectivity. Your point about "cost of running a city grows" has many factors, too. A huge one is the unfunded, ridiculous, seemingly corrupt pensions. Citizens will, at some point, have to foot the bill. If I saw that in my future I'd move out, too. Those cities/states are looking for a Federal bailout which I hope never happens and, if it does, requires the city to fix their systems.