Over population is not about population density. It is about available resources. In much of the "unused" land you refer to there is no water. If there is no water, there is no industry to provide jobs for people living in that area. Under much of the "grasslands" to which you refer, flows the Ogallala Aquifer. Which is being rapidly diminished by the demands of the population we have now. Now, do not cast your beady eyes upon the Great Lakes as source of water for the rest of the country; we do not wish the Great Lakes to become the Great Swamps.In addition, the planet is not overpopulated; we have plenty of pristine, unpopulated land (much more than we currently use - Brazil alone only uses half of it's land) to use at our discretion.
Let's look at the US:
In 2002 20 percent of the US was cropland and 3 percent of the US was urban areas - the other 77 percent was forest, grasslands, "special use land" and miscellaneous land.
In other words, we don't really use 77% of the land in the USA.
Get a grip.
http://en.wikipedia.org/wiki/Ogallala_Aquifer
Maybe you can live on Soylent Green. But the those of us who understand what overpopulation means do not want to. Please get a grip.