Utilize the input text as a context reference, choose the right relationship between millions and Chicago from the options. Options: product/material produced, manufacturer, distributed by, industry, position held, original broadcaster, owned by, founded by, distribution format, headquarters location, stock exchange, currency, parent organization, chief executive officer, director/manager, owner of, operator, member of, employer, chairperson, platform, subsidiary, legal form, publisher, developer, brand, business division, location of formation, creator.

Sea-Tac is much smaller than airports such as those in Denver and Chicago that host tens of millions of passengers.

Answer: location_of_formation