I use XFCE because I run debian in a VM inside MacOS and I don't see much reason to run two RAM guzzling window managers (I have lots of RAM so I can run scientific code that often needs 4-5GB at once, mostly for python code I intend to optimise when I have time). At 70MB by this measure, it seems like I made roughly the right choice for a window manager that leaves me lots of other space free for 'real' data (but still loads enough of GTK to run Eclipse as my dev env.)