Of course this country is overcrowded. While there are many unsettled areas, most of them do not have the water supply to support a town. Back in the early days there was a need for more immigrants, but not any more - certainly not immigrants who are from hostile countries and refuse to even learn English but demand all kinds of special services.
I know of many historical and scenic areas that have been destroyed for tacky housing developments. The population has been growing and growing (if you don't believe me, just look up the statistics) while the amount of land remains the same and certain resources are dwindling.
An interesting discussion of overcrowding can be found in "The Pump House Gang" by Tom Wolfe, Ch. 15 (O Rotten Gotham - Sliding Down into the Behavioral Sink)
Here's a map of U.S. population density, you can see all the yellow out west; that's all not populated.
http://www.census.gov/dmd/www/pdf/512popdn.pdf Furthermore, without immigration, the U.S. population would be aging at a considerable rate and would be depopulating just like Europe because most American women don't want to stay at home with ten kids.
As for the so-called "tacky" suburbs, those are populated American citizens (generally of the white trash variety), not immigrants. Moreover, the issue of urban sprawl has more to do with improper planning on the part of municipalities than it has to do with too many people. City and county governments are too busy counting the $$$ from developers to understand that more people = more schools, more roads, more hospitals, etc.