I have the same problem on my workstation, after a couple of days gnome-shell ends up eating 800+ MBs. I believe it's a memory leak there are several seemingly different bug reports both on MInt Launchpad (e.g. https://bugs.launchpad.net/linuxmint/+bug/930402
), and Gnome bugzilla (e.g. https://bugzilla.gnome.org/show_bug.cgi?id=642652
Some simple fixes to release that memory include an X-restart, or simply Alt-F2, type r, and press return. Note that these are not true solutions to the underlying problem but just ways of avoiding larger problems.
If anyone comes by a better description/explanation of the problem, or a better solution, please do tell..