This doesn't jibe with the current House and Senate, where the equal-population represented House has a substantial Republican edge and the Senate has a majority of Democrats.
Edit: btilly below is right, the House isn't technically proportional representation, it's equal-population districts. Still, state by state representation doesn't seem to be breaking reliably for Republicans.
If the House is "proportionately representative" then how did Democrats win substantially in the popular vote in congressional races, while Republicans won the house by a solid margin?
In a perfectly gerrymandered system, the party in power can win using only 25.1% of the vote - and that's if all districts have equal population. You can get by with even less if the districts have imbalanced populations. In that "perfect" system, you draw the districts so that your detractors are 100% of the population in the districts you know that you're going to lose, and your supporters are 50.1% of the population in the ares you want to win.
In real life, of course, it's messier and you can't draw the lines quite so perfectly, but here's a link to the districts near Chicago - look how oddly-shaped they are, especially as you get closer to the city center. (They'll probably change again next election season, doubly so if the other party gets control.) http://www.govtrack.us/congress/members/IL
The same way that the reverse used to happen when the Democratic Party caucus had a majority in the House of Representatives in the 1980s. In those days, there was much higher turnout in Republican-leaning districts, but there were fewer of those districts, because Republican voters were concentrated into just a few congressional districts in most states by state legislatures that mostly had a Democratic majority. Now that the tables are turned, people from the other side of the aisle notice this as a problem, but always it takes "tide" elections to switch majority control of state legislatures, which then lock in place (until the next tide) advantages for the majority party in the drawing of electoral districts. This too shall pass.
The Republicans were able to control the house while losing the national congressional popular vote by being better able to redistrict their congressional seats in ways that were favorable to them.
This is why it is important which party controls State legislatures, those legislatures favor their own party when they draw the map.
Edit: btilly below is right, the House isn't technically proportional representation, it's equal-population districts. Still, state by state representation doesn't seem to be breaking reliably for Republicans.