A U.S. senator represents a state as a whole and its interests in Congress. Since 1913, members of the Senate have been elected directly by the general population of a state, but before that, state legislatures appointed senators.
In terms of geography and population, there is a sizable difference between each state. The United States features a bicameral legislature, which seeks to eliminate the tension between equal representation for each state and equal representation for each person, regardless of state. This feature of the U.S. legislative system is known as the Great Compromise. There are two senators per state to give each state an equal say in the affairs of the federal government.