As of March 2021, there are 100 U.S. Senators (as there has been since the admission of Hawaii in 1959), 50 of whom are Democrats (or independents who caucus with the Democrats), and 50 of whom are Republicans. (Democratic Vice-president Kamala Harris can cast a tie-breaking vote, which gives control of the chamber to the Democrats.) However, the states represented by Democrats are altogether larger in population than those represented by Republicans. This graphic illustrates how.
Population figures are from 2019.