All Start Screen tiles have shrunk on XPS 12 after upgrading to Windows 8.1

When I upgraded to Windows 8.1 on my XPS 12, all of the live tiles on my Start Screen shrunk. This meant that there is now an additional fourth row of medium-sized tiles on the Start Screen. The text in the tiles is also a lot smaller to read.

I don't really understand why this has happened. All of my drivers are up-to-date. (And just to clarify, I'm not referring to the new feature in Windows 8.1 where you can set individual tiles to be smaller or larger.)