“I still do not see why Champaign/Urbana, IL, is not there. It is no smaller than Ann Arbor.”
They used Metropolitan Statistical Area population. According to the U.S. Census, the Ann Arbor MSA has a population of 368,000. The Champaign/Urbana MSA has a population of 231,000. That makes Ann Arbor approximately 60% bigger than Champaign/Urbana.