Grant Park (Chicago) is 15 times larger than Millennium Park.
Area: 1 km2 calculated
Grant Park is a large urban park in the Loop community area of Chicago. Located in Chicago's central business district, the park's most notable features are Millennium Park, Buckingham Fountain, the Art Institute of Chicago and the Museum Campus. Wikipedia
Area: 0 km2 calculated
Millennium Park is a public park located in the Loop community area of Chicago in Illinois, US, and originally intended to celebrate the millennium. It is a prominent civic center near the city's Lake Michigan shoreline that covers a 24.5-acre section of northwestern Grant Park. Wikipedia