WASHINGTON -- The average size of the nation's 100 most populated cities is about 168 square miles, more than triple the size in 1950, Census 2000 figures show. Cities in the South and West led the land grab.

"Most growth in cities has not been in the building of big apartment blocks, but cities capturing a new subdivision" on their outskirts, said Robert Lang, director of the Alexandria, Va.-based Institute for Metropolitan Research of Virginia Tech. In other instances, cities have annexed empty tracts for future development.

City officials, eager to add to their tax bases, often boast of services, such as fire and police protection, garbage pickup and street maintenance, when they move to take over a populated area.

Suburbs grew after World War II as Americans sought to own homes on bigger lots in quieter neighborhoods. Banks, supermarkets, retail stores soon followed.

In 1910, the average size of the 100 cities with the largest populations was 31 square miles. It grew to 50 miles in 1950 and 168 in 2000.