I don't actually remember who brought the case, but I mentioned it because it was merely the most recent decision. But I'll underscore what I wrote earlier--it was a 9-0 decision because the constitutional language is pretty clear. If the framers had meant "citizens," they wouldn't have written "residents." The opinion is quite straightforward and worth your perusal.
Now, since the number of representatives is capped, you are correct in suggesting that it's zero-sum. A rep added in one state means one removed from another. This may not pass common-sense muster with you, but neither does the alternative, really. The government at the federal, state, and local level is responsible for securing resources for their state/district/municipality. Demand for those resources is determined by the population in those various subdivisions. Is it "unfair" for one area to get more representation than another based on a large disparity in population but a smaller disparity in citizen population? Arguably, yes. Is it also "unfair" for non-citizens to be contributing to the economy and the local tax coffers and have no franchise or representation at all for a potentially lengthy passsge to citizenship? Arguably, yes.
Our bicameral system already has a huge check in place to prevent small states from being rolled by more populous ones. Legal remedies to check proportional representation at the expense of states whose populations are growing seem rather anti-democratic to me, I guess.
If you take the position that I suspect thethe is getting at--that the framers didn't foresee illegal immigration on the current scale and didn't specifically intend the current status quo, I can only throw up my hands and say that they didn't foresee concealable semiautomatic handguns or AR-15s with high-capacity magazines, either, nor specifically intend our individual right to own them outside the context of ye olde well-regulated militia. If you want to join us on the dark side, I say the more, the merrier.