A decade ago, that was a lot of RAM for a desktop environment. In the late '00s, I remember Ubuntu with Gnome 2 using around ~128 MB of RAM right after boot.
What happened? Most DEs aren't that much more complicated than they were a decade+ ago. Is it the array of supporting libraries (Qt and Gtk) that get loaded into memory? I could see that being a problem since even the "lightweight" DEs like XFCE and LXQt rely on them heavily.
There's probably also things like unicode data (ICU is like 20MB), more daemons (WiFi, rendezvous, Bluetooth, etc.), and I think C compilers have generally optimised for speed at the cost of code size over time.
There is also the case to be made that RAM is made to be used, and that half a gigabyte is a perfectly reasonable amount to allocate to a snappy desktop in a world where even the lamest new computer has at least 8.
My point is that that's a recent-ish change, the lightweight DEs used to use less than 150 MB of RAM, and it's rather surprising how much the gap has closed. I don't know any truly lightweight DEs; to get small you have to use a tiling WM like i3 or sway.