Immigrants kept the largest urban counties in the U.S. growing last year. Core counties in the Houston, Miami and Phoenix metropolitan areas grew more than any others in the country primarily because of people moving in from outside the United States, according to population estimates from the U.S. Census Bureau released Thursday. Without the international migration, Harris County, Texas, Miami-Dade County, Florida, and Maricopa County, Arizona, would have had nobody moving there last year.