The argument is that with more representatives you take the money out of elections as companies and individuals cannot influence that many people.
Interestingly, that was one of the arguments the Progressives used to forward the 17th Amendment—bribing/buying the votes of the entire electorate would be impossibly expensive as opposed to bribing dozens of state legislators. We can all see now how big money plays no part in Senatorial elections.
Another argument is that there would be more politicians to cover each other while all screw over the citizens.