During the Constitutional Convention (1787, one of the issues of disagreement was how would states be represented in Congress.
People from larger states like Virginia, Massachusetts or Pennsylvania thought it would be unfair that each state were given the same number of representatives regardless of population, and argued that the best way to ensure equality among people within the states was to divide states' votes according to population, so the most populated states would have more votes.
This would affect the number of each state delegates sent to Congress and the degree of power of each state in the new government.
Also, larger states have more diverse population with their multiple needs. If the states only sent one delegate, they feared it could meet their needs.
Another reason has to do with taxation. Larger states would pay more taxes than smaller ones, because they had more human capital creating wealth. So relying on the promise of the American Revolution (taxation and representation) they argued that they should be represented in the Congress accordingly to the taxes they paid.
