[Lawrie]

Hi Mike,
I've had a few minutes to play around. 
With MS set to NOT use full screen nor to hide the navigation bar:

1. Open a collection and choose a song
   
2. Rotate to landscape
   
3. Go back to library page (I use the Left arrow/triangle in the nav. bar at the bottom)
   
4. Select the collections tab and there's a very good chance the collections list will be in the large font - or even a mix where the top few collections will be normal, and then the font will get bigger.  NB there only seems to be 2 font sizes, Normal or Big.
   
5. Rotating back to portrait doesn't seem to change things unless you do it BEFORE going back to the library page (step 3), then it seems to be inconsistent.
   

Some points to note.
The font size only changes in the listing section of the library display, the filters and tab names etc. are unaffected.
I use the tablet "upside down"* with the USB port at the top in portrait mode so I can access the port if needed for a USB pedal or charger.  The tablet is used on a music stand, not a tablet holder.
It only seems to happen if I need to use landscape for a chart and go back to the library before rotating back to portrait
I always rotate clockwise to get to landscape and anti-clockwise to get back to portrait due to the way the case works.*
I don't think these steps work every time, but seem to most of the time.

* This is probably irrelevant.