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