[kubuntu] Everything is too small

Some desktops have tools to make adjustments to the size of titles bars and menus, so it can work better on a high dpi display.

Vanilla Ubuntu, uses Unity, and it can adjust those settings on 'Displays'. Since it is based on Gnome, I imagine the latter have a setting too.

As far as I can tell, KDE does not have such feature, although you may try this: Kubuntu and HiDPI Screen.

However, KDE plasma 5 have something:

System Settings → Display and Monitor → Display Configuration → Scale Display

Hope it helps. Let us know how it goes.
Regards.