Infographic map depicting US population change between 2010 and 2020. Each coloured unit represents a US County. The county with the biggest population increase is Maricopa County in Arizona (753,898 increase). The biggest drop in population was in Cook County, Chicago (90,693 decrease). There are a number of factors that determine how much a region?檚 population changes. If an area sees a high number of migrants, along with a strong birth rate and low death rate, then its population is bound to increase over time. On the flip side, if more people are leaving the area than coming in, and the region?檚 birth rate is low, then its population will likely decline. Data from US Census Bureau 2021.