A US Census report in 2012 (PDF) singled out Chicago as the metro area with the largest numeric increase in “downtown” population between 2000 and 2010. For purposes of the report, the Census Bureau defined “downtown” as the area within a 2-mile radius of the location of the city hall in the principal city in a metropolitan area.
A recent study that looks at urbanized areas beyond just the downtown core of metropolitan areas presents a different take on urban growth. Among 52 US metro areas, Chicago had the highest share of its population growth, 148.2%, occurring in areas that were functionally suburban and exurban. The study utilized the City Sector Model, which classifies ZIP Codes based on how closely they hew to characteristics of an urban core.