Well then wouldn't Lake Superior be a lot larger, since it has about 9k more sq. mi. of surface area than Lake Michigan? Or are you counting Huron as well? Hydrologists consider Michigan and Huron to be one lake.
Lake Baikal is the world's largest freshwater lake in terms of volume. It contains about 5521 cubic miles of water (23,000 cubic kilometers), or approximately 20% of Earth's fresh surface water. This is a volume of water approximately equivalent to all five of the North American Great Lakes combined