On my system, (length (x-list-fonts "*")) is 3007, and this function:

(defun bloat-font ()
   (interactive)
   (let ((fonts (x-list-fonts "*")))
     (while fonts
       (condition-case nil (set-frame-font (car fonts)) (error nil))
       (setq fonts (cdr fonts))
       (redisplay))))

causes Emacs (default GTK3 build) to grow up to ~360M RSS, mostly
because a lot of font resources aren't freed when font objects are
reclaimed.  Proper reclamation reduces RSS to ~150M.  This is the
most giant memory leak I've ever seen.  I'm trying to fix this by
adding simple finalization support for Lisp_Vectors; comments are
welcome.

Dmitry