You can't generalize population density across the entire continent like that. Montana and Wyoming have a wildly different population density than California or New Jersey and they're all in the contiguous United States. Even just looking at the region east of the Mississippi to between the Mississippi and the Rockies, shows a striking difference in population density.

Also, Spain is like 1/16 the size of the US with mostly a singular population center in the literal center of the country. And, the Paris metropolitan area is comparable to the tri-state NY Metro area and a fraction of the size of the surrounding region from NY to D.C., which itself is a tiny fraction of the area we call the East Coast.