The default iconsize of the application toolbar of LibreOffice's user interface is "large" instead of "Automatic" (or "Small").
Current default state (as-is state):
LibreOffice -> Preferences -> LibreOffice -> View -> User Interface -> Icon size and style -> Large
LibreOffice -> Preferences -> LibreOffice -> View -> User Interface -> Icon size and style -> Automatic
*** This bug has been marked as a duplicate of bug 30525 ***