Over a 20-year period, the 100 largest Urbanized Areas examined in this study sprawled out over an additional 14,545 square miles. That was more than 9 million acres of natural habitats, farmland and other rural space that were covered over by the asphalt, buildings and sub-divisions of suburbia. And that was just for the half of Americans who live in those 100 cities. A major controversy in the efforts to halt the rural land loss is whether land-use and consumption decisions are the primary engines of urban sprawl, or whether it is the nation’s continuing population boom providing most of the power driving the expansion. A careful analysis of U.S. Census Bureau data found that the two sprawl factors share equally in the blame: per-capita sprawl and population growth. Immigration is the largest cause of population growth.
Analysis of U.S. Bureau of the Census Data on the 100 Largest Urbanized Areas of the United States by Leon Kolankiewicz and Roy Beck, March 2001"