That's why one is called The U.S. House of Representatives...get it? Representatives? They represent the constituents of a particular state at the federal level. More constituents = more federal representatives.
Until the 17th Amendment was passed in 1912, the U.S. Senators were chosen by each state's elected state representatives and senators. The people voted for their state representatives and senators (the state legislature) and that state legislature would elect the US Senators to go to Washington D.C. The U.S. Senators were not intended to be simply cardboard cutouts of the will of the people in electing their U.S. representatives (as it is now). When problems and bickering arose around 1840, the solution that was eventually achieved was to put the election of senators in the direct hands of the people (and thereby eliminate the voice of much of the small population centers that was originally brilliantly provided by the original constitution).
For example, in Oregon, the large metropolitan population centers in Portland and Eugene can outvote most of rest of the state, and usually do. They vote democrat. However, those people in the metro areas (and their elected reps) don't always have the best interests of the more rural areas in mind. Those areas vote Republican. At the state level, that can be addressed. At the federal level...not so much. If Portland and Eugene want federal strawberry ice cream, and the rest of the state wants federal chocolate...well...sorry, but the entire state must eat federal strawberry. The electoral college is designed to make sure that the huge population centers don't outstrip the rest of the smaller individual votes, so that the 'masses' of chocolate lovers can be heard! In the same way, the U.S. Senate is meant to make sure the huge population center's U.S. Representatives don't overwhelm the legislative branch of the Federal government - the good ol' "Checks and Balances" system.
