The population of the Chicago metropolitan area i.e. "Chicagoland" is 9.5 million (almost the size of Michigan). So 6.7 million people are technically not in the city.
A significant fraction of them commute to downtown Chicago for work.
That's really a lot, 70%! How big of a portion of them are commuting to downtown?
Anecdotally, as someone whose walk to work (from other transit) takes them against the flow of people coming from Union Station: it's a lot of people indeed.
September 2018 Metra ridership report: https://metrarail.com/sites/default/files/assets/planning/ri...
All reports: https://metrarail.com/about-metra/reports-documents/operatio...
Shenzhen has twice as many people as Chicago. So, if we extrapolate, the amount of people commuting will double for Chicago if Chicago were to be turned into Shenzhen.
That tells that much, much, more people in Chicago work outside of urban centre, or the portion of population who is working is simply less.
The U.S. has 1/4 of China's populations, Chicago is the 3rd largest metro area in the U.S. It has a population comparable to Qingdao, but it's relative influence in the country is similar to Chongqing or Hangzhou.