I solved my own problem, but I'd like to post the solution for anyone else who may come across this issue, as I don't believe I'm the only one who's ever had this problem.
The issue is that when running Windows via Boot Camp, the Retina display is reported at its full resolution (2880x1800). This results in really tiny things on screen until the proper DPI is set manually or the resolution is lowered (leading to a muddy/fuzzy desktop). Regardless of the set resolution, perhaps due to DirectX allowing Windows display settings to be bypassed, full-screen programs can grab the Retina display's native resolution and adjust to fit it in all its eye-pleasing glory.
The problem here is that the (as of March 2015) Nvidia GT750M chip has nowhere near enough muscle to power almost any game at any reasonable level at the highest resolution (for instance, in a recent test, even on the lowest video settings Crysis 2 ran at a paltry 15ish fps). At 1920x1200 the GPU can still choke on some things, although for the most part I've found most recent releases can hit 60fps at medium-high settings.
The sensible thing to do here is of course to lower the resolution to 1920x1200. When you do that, some games get it right: scale to fit the entire screen; but most games simply throw up some nice black bars and you're left with about half the screen to play in. No amount of playing with Windows' display settings will fix this.
Right-click on the desktop and select the Nvidia Control Panel. On the left column select "Adjust desktop size and position", and in there you'll find a checkbox option labeled "Override the scaling mode set by games and programs". Selecting this will stop games from displaying lower resolutions such as 1920x1200 at 1:1 ratio, and instead allow the GPU to fill the entire display.
Note that this solution is specific to Retina MacBook Pros running Nvidia chips; I have no idea if this will work for other displays on Nvidia chips having similar issues, although I have a strong suspicion that it will.