By size, you are referring more specifically to area. Area while neglecting population is inversely proportional to population density¹. But even apart from that – how does that support the claim that it’s sensible to disregard cities and just look per capita nationwide? NYC should be compared as a single whole city against other cities of comparable population density. Area does not matter as an independent variable on its own. What would the point be to blur NYC into a nationwide track per capita?
BTW, NYC has a subway system. I’ve used it a few times and it was not even close to being overcrowded but maybe I had lucky timing. Are you saying more track is needed there?
¹ population density: heads per m²
The nationwide fuckup in the US is zoning rules that block commercial venues from residential regions, which means people cannot step outside their front door and get groceries in a 1 block walk. People are forced to travel unwalkable distances to reach anything, like food and employment. Which puts everyone in a car. Which means huge amounts of space is needed for wide roads and extensive car parking, generally big asphalt lots, which exacerbates the problem because even more space is wasted which requires everything to be spread out even more, putting resources out of the reach of cyclists. Making the city mostly concrete and asphalt also means water draining problems where less of it makes it into the soil and groundwater, and it means the city temp is higher because of less evaporative cooling from the land mass (Arizona in particular).
This foolishness is all done for pleasant window views, so everyone can have a view of neighbors gardens instead of a shop front.
Europe demonstrates smarter zoning, where you often have a shop on the ground level and housing above it. You don’t need a car because everything is in walking or cycling distance. But you more likely have an unpleasant view.