KDE can be considered heavy only if your idea of a desktop use is to launch it and stop right there.

But normally after that you launch apps and that’s where the magic happens: it is so integrated, apps barely add any more RAM usage on top.

So instead of comparing DE x and y, compare what a desktop actively used looks like: browser? office suite? file manager? drawing app?

Only then will you be able to compare you RAM usage from one DE to another. Everything else is comparing cars fuel economy when they’re all idle.