Um... Do you mean a full size Walther P99? Because if you put that next to your Glock 17, you'll see that it's really no smaller (just nicely curved instead of a, well, block). I know, looking at them separately you'd swear there's a size difference, but go ahead and compare them directly. You'll see what I mean.
And since the P99 uses 16 round magazines (at least the gen 1 mags are 16 rounders), don't you only lose 1 round in capacity?