While I don't see the use of such a thing, in the mean time you could hold down the Ctrl key and give your mouse wheel an upwards tolchock. That will make the editor's font size tiny (for whatever it's good).
Seriously though, we're talking of source code here, not of images, right? So what's the actual benefit of seeing things in thumbnail size? Aren't bookmarks or find/jump to implementation a whole lot more useful than trying to guess from a thumbnail where you want to go?