Is it possible to reduce the height of the UI?

It’s freaking huge, in case no one has noticed. Toolbars and menus alike.

Wath’s your OS?

And in the case of Linux, *BSD or other Open Source platfom, the desktop environment or GUI.

My OS is Windows 7.

I have not W7 now on W10, but I think you can find how to do in Panel control/Appearance…/Screen

Seems as you have to relocate the toolbars.

  • Click the menu Tools, then Options.
  • On the left expand LibreOffice then click View.
  • On the right, under User Interface.
  • Icon size and style only changes the Icons.
  • Scaling changes the menu.

Update: 22/Jan/2016

The height should automatically change to fit the text.
Since it does not then it is likely your LibreOffice user profile is corrupted. Before resetting the whole LO user folder, try resetting the file registrymodifications.xcu

For Windows 7 the location is

%appdata%\libreoffice\4\user   (LibreOffice 4 & 5)
%appdata%\libreoffice\3\user   (LibreOffice 3)

From the Wiki - Resolving corruption in the user profile

In many cases it would be sufficient to only rename/(re)move the file registrymodifications.xcu within the user profile to reset all options to their default values, instead of moving the entire user profile directory. Doing so preserves all user installed extensions, gallery, auto-correction entries and auto-text and so on. But of course this would not help if, for example, one of the extensions or some other corrupted file was the culprit. You may want to experiment with that.
But mind that this has effect on registration of macro’s too, and makes that custom colors are removed (standard.soc overwritten).

That just seems to change the size of the text of menu, not the actual height.

The usual suspect in such a strange case is the user profile. It might be corrupted (by a crash/blackout).
Please consult this wiki page.

It was like that from first installation though…

Did you try nonetheless?