Again, you're mistaking a Senator representing a state vs the people in a state. The Senator represents the state, no matter if they got 51% or 100%.
What makes you say that Republican senators represent a minority of the population?
There is no real way to determine the population in each state represented by a party other than votes. The presidential vote tallies (the only truly national vote) are the closest we have to this. Numbers in the Senate are fairly close to the popular vote (not a coincidence).
