According to the most recent United States Census, Los Angeles County in California is the largest county in the nation, with a total population of 10,150,558 – an increase of 3.5% since 2010. Chicago's Cook County follows and, although very large in its own right, is half the size of Los Angeles County with 5,231,356 residents. Harris County in Texas, Maricopa County in Arizona, and San Diego County in California make up the rest of the top five – all have populations in excess of three million.
Statistics from 2018 show that a number of counties have had substantial population growths since 2010. Fort Bend County in Texas tops the list, with its population having grown from 590,286 in 2010 to well over a population of 764,000 in 2018 – an increase of 30.8%. Three other counties in Texas are also in the top five counties with increased populations. These are Denton County (with an increase of 26.2%), Collin County (24%) and Travis County (19.7%).
Conversely, while Wayne County is the most populous county in state of Michigan, it has seen its population decrease by 3.7% since 2010, from 1,815,069 to 1,756,598. Counties in two other midwestern states, Missouri and Ohio, have also had population decreases, although both are more modest. Ohio's Cuyahoga County has decreased from 1,278,200 to just around 1,248,000, representing a reduction of 2.5%, while St. Louis County, Missouri's population has decreased from 998,681 in 2010 to 998,216 in 2018. Just one county, Hartford County in Connecticut, shows nearly zero population change, with census figures reporting a population of approximately 895,500 in both 2010 and 2018.