But kde applications use even more space with the title bar, menu bar and tool bar. Even if you disable the toolbar and menu bar, there's much more padding in kde applications resulting in the content area being significantly smaller. I just compared Kate to gedit and the new gnome text editor and it's not even close. The gnome applications are much more compact.

I've been seeing people complain about header bars being "huge" for years, but every time I actually do a comparison, the header bar application turns out to be more compact than the alternative.

The only issue with gnome in that area is their antagonism towards themes, as themes can easily fix any size possible problems, but the newest default theme is quite reasonable. I used to have custom CSS to shrink the header bars, but it's no longer necessary.

That being said I recently switched to plasma as well, as gnome's forced Wayland transition resulted in way too many workflow issues and bugs. But I just configured plasma to work like gnome-shell and I'm continuing to use gnome applications.