Martin Gräßlin has written at length in a new blog post about using KWin with other non-KDE desktops. It is possible and the footprint isn't too bad... It does require also having installed the KDE libraries and a few run-time modules, but it doesn't mandate the use of the Plasma Desktop or other KDE packages. Martin argues that using the KDE's window manager on other desktop environments won't be too much disk space and all around shouldn't be a bad process to setup these days. In Martin's post he also covers some basic memory usage numbers and other factors to consider.
Martin concluded, "Given that I can only recommend to give KWin a try and not to discard it because it is from KDE and might pull in some dependencies. Evaluate by the features we provide and you want to use and not by some random number on your hard disk or your memory usage."