"The United States has a total land area of nearly 2.3 billion acres. Major uses in 2002 were forest-use land, 651 million acres (28.8 percent); grassland pasture and range land, 587 million acres (25.9 percent); cropland, 442 million acres (19.5 percent); special uses (primarily parks and wildlife areas), 297 million acres (13.1 percent); miscellaneous other uses, 228 million acres (10.1 percent); and urban land, 60 million acres (2.6 percent)." http://www.ers.usda.gov/publications/EIB14/
Let's just look at that again, shall we? Urban land = 2.6 per cent. In other words you could double the developed land and it would still be just over a twentieth. You can fit your urban land into your parks and wildlife areas five times over. More than half of the US is either pasture, range of forest-use land (yes, I know, wrong kind of forest - there's no pleasing some people.)
But what is the trend, you might ask? Well it is true that Urban land grew by ten million acres between 1982 and 2002, but over the same period, Special Use (that's parks and wildlife areas etc.) grew by 27 million acres. The major decline being in cropland and pasture. See the table at http://www.ers.usda.gov/publications/EIB14/eib14b.pdf