Actually, that's not so, or not stated very correctly. You could stack rounds single-stack style and each would take up the full vertical height of the case head (0.394"). Or you could lay two rounds exactly side-by-side and TWO rounds would take up 0.394" of vertical height. In between those two extremes, you could vary the mag width almost infinitely to allow the rounds to spread out more or less, and thus, together, take up somewhere between 1 x the number of rounds times 0.394" and 1/2 that height.
Here are a few drawings I did to illustrate the point.
Using the thickness of the walls and ribs of an xDM's magazine (0.066"), I get a total outside width of 0.526" and the width of the cartridge heads (0.394") inside. In this magazine, I get four rounds in 1.62" of vertical height.
I could in theory make a magazine with the same wall/rib thickness that let the rounds sit fully side-by-side. That would require an outside dimension of 0.920" and an inside dimension of around 0.788" -- twice the diameter of one cartridge. I get EIGHT rounds in that same 1.62" of vertical height. (Though it probably won't feed very well.)
If I play with things a bit, I can get other whole number integers of rounds into that same vertical height. Here's five. The mag has to be 0.790" in width, and 0.658" inside.
And, of course, we need to see what an xDM mag really looks like. Here it is. It is 0.864" in overall width, and 0.732" inside. That gives an even stack of seven rounds in that same vertical height.
That arrangement really reduces the amount of dead space inside the magazine (I think most double-stack mags are optimized this way to some degree) but it doesn't actually eliminate it. One round doesn't actually touch the next round on top of it, but bottoms out between the mag body rib and the lower quadrant of the round beside it. There's a smidge of room left between rounds on top of each other. It isn't much -- about 0.013" -- and it would take a lot of stacked rounds to "build" room for another in the mag just by removing that slop.
I can't squeeze another full round into 1.62" by letting the seven round stack gap open a bit wider, but I could make those seven rounds
shorter, and when you're getting up into the double digits, you could quickly come up with enough room.
These little increments of gained volume are kind of funny in how they work. I have friends who play with open guns for USPSA who will buy big piles of parts to put together STI or PARA mags that hold crazy numbers of rounds -- over 30 for some. And the tiniest differences gain or lose them extra rounds. In fact, what works for one gun or mag body might not work for another. They're basically putting them together and seeing what they can get. Might be 27, 28, 29, plus. Depends on the spring, follower, mag body, base-plate, feed lips, and pixie dust, I think. ... And then they have to get these $150 monstrosities to RUN!
I'll stick with the much more sane xDM 19-rounders!