In my experience, the order of these four DEs based on resource usage is exactly the same as what you wrote in your opening post.
I think there was a recent report floating around bench-marking Plasma very close to XFCE now which was the shocker, and I think was lighter than Mate. I will see if I can find it. Plasma has come a long way.
Edit to add: Although I could not find the benchmark report I was thinking of, here are a couple comparing XFCE which has always been known as the lightest, to KDE;
KDE Plasma Desktop vs Xfce Desktop - Resource Usage Comparison | 2020 Edition
Forbes: These Xfce Versus KDE Numbers Reveal A Shocking Surprise About Linux Desktop Environments
1 Like
I always thought XFCE was lighter than Plasma, althout not by much.
1 Like
I did as well.
After reading & watching that video, maybe i should have opted for EOS Plasma. 
It was until xfce moved to gtk3 and plasma 5 came out. KDE is definitely lighter these days.
If you want light And still an actual DE. My LXDE setup a few months ago base lined at just over 300mb of ram.
3 Likes
How does Mate fair these days in lightness? 
1 Like
on same hardware & video card and screen ,
Xorg use 130Mo for Xfce ( Gtk3 + Gtk2 ) , 80Mo on Mate ( gtk3 )
2 Likes
Openbox. Or maybe i3
Puppy linux
1 Like
Openbox and i3 are window managers, not desktop environments. A window manager is just one of many components of a fully featured desktop environment. It is possible to run it without the rest of the components, but in that case, it is not meaningful to compare the use of resources.
Puppy Linux is a distribution, which, by default uses JWM as its window manager.
4 Likes