Modern WMs and desktop shells use way more memory because they cache more bitmaps (we now render text as bitmaps and composite them, and the drawing styles/themes are now much more than flat gray fill with the odd non-antialiased line), and because the bitmaps are far larger due to much higher resolutions and bit depths.
A single uncompressed 1080p 32-bit (24-bit with 8-bit alpha) wallpaper is 66MB of RAM. A similar single 4k wallpaper takes 265MB RAM. All those fancy-looking high-res 32-bit icons also take huge amounts of RAM.
I'm an idiot, you're right. There, it's on record. I claim under-caffeination. Still, the basic point still stands, even if my numbers need to be divided by 8. Desktops use more memory because, as users, we demand fancy true-color graphics, high resolutions, and anti-aliasing that we didn't have in the Windows 9x era.
A single uncompressed 1080p 32-bit (24-bit with 8-bit alpha) wallpaper is 66MB of RAM. A similar single 4k wallpaper takes 265MB RAM. All those fancy-looking high-res 32-bit icons also take huge amounts of RAM.