Well, you see, there's a popular brand of computers that don't have separate VRAM and have twice the display resolution of everyone else.
Luckily, windows aren't always fullscreen and so the memory usage is somewhat up to the user. Unluckily, you often need redundant buffers for parts of the UI tree, even if they're offscreen, eg because of blending or because we want scrolling to work without hitches.
But window buffers are usually in VRAM, not regular RAM, right? And I assume that their size would be relatively fixed in system and depend on your resolution (though I don't know precisely how they work). I would think that the total memory taken up by window buffers would be relatively constant and unchanging no matter what you have open - everything else is overhead that any given program ordered, which is what we're concerned about.