I don't have the magazine anymore, but I once read an article by Chuck Taylor where he was describing the many tortures he had subjected his Glock 17 to over the years...
Anyway, one of his comments that I took particular interest in was that he noted that GLOCK magazines (can't speak for the others) did tend to wear out quickly (with use I'm sure) if you kept them fully loaded. He said that (again, his experience), if he kept his magazine downloaded to 15 rounds, or installed a +2 baseplate and used 17 rounds, his mags did not wear out, even with YEARS of use.
This information is pretty much in line with what JohnKSa said about double-stacks, and also what I've noticed when disassembling mags, removing the springs, inserting the follower and rounds, then looking to see how much space is left for the spring. With the Glock squeezing in normally 2 extra rounds into the same size pistol, I bet those springs are pretty darned "coil-bound".
For my wife's "bedside" Glock 19, I did as Chuck suggested and installed a +2 baseplate and only loaded 15+1. For my bedside USP45, I don't download, but I don't "top off" my 12 round mags. I figure as big as those .45 rounds are, that leaves me enough room.
I was brutally insulted years ago on this website for stating my magazine practices, (something about "what do you load in your P32, 2 rounds, 3? Why not just carry a derringer you moron") so before it happens again, I do NOT download my single stack pistols and I DO top them off as those springs don't appear to be as compressed anyway... The bigger the mag, the longer the spring, the higher the capacity, the more consideration I give to downloading by 1 or 2 rounds.