Hyde Park, London is 15 times larger than Millennium Park.
Area: 1 km2 calculated
Hyde Park is one of the largest parks in London, and one of the Royal Parks of London, famous for its Speakers' Corner. The park was the site of the Great Exhibition of 1851, for which the Crystal Palace was designed by Joseph Paxton. 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