The distribution of people across the United States is increasingly dispersed. The total number of people living in the United States is growing slowly - less that 1 percent per year - and the land area is essentially unchanged. But the population distribution is changing from relatively clustered in the Northeast to more evenly dispersed across the country.
Remember, concentration is not the same as density. Two neighborhoods can have the same density of housing but different concentrations. In a dispersed neighborhood each house has a large private yard, whereas in a clustered neighborhood the houses are close together and the open space is shared as a community park.