No, it's nearly :-) as tall as a 1080p screen. It's 810px (edit: modulo some scaling?) but you also rarely get the full 1080p screen height on a Plasma desktop, since you've got a big panel at the bottom of the screen. Hence "nearly".
> Finally, the things you mention are not related to the theme at all, but instead the layout of individual applications.
Widget size, padding and margins are all part of the theme. I use QtCurve (which is pretty wasteful, too, though nowhere near as bad) and it's far more efficient to use.
A theme that makes better use of available screen space would allow one to keep the toolbar around, and make better use of search features (yes, the search panel is usually not folded out, but if you have to fold it out, search, then scroll or fold it back in to read around a search results, that's not exactly useful).
If you need to disable UI elements to make an application usable, that's not a good design.