[itsme]

YESS - that's it.
After copying the database the page dimensions are -1 / -1 again. 
I think we had the question before, if the page dimensions are stored in the database or not? And the answer was yes, they should be in the database?
Correcting width and height to the original values fixes the displayed lyrics size so that it's the same on both devices. I used "apply to current file" because I'm still experimenting with different settings.

So it seems that page dimensions, at least when applied to current file only, are lost somehow when I copy the database.

It's intentionally 800 x 1208 (not 1280) and 1860 x 2354 (not 2480). That's what "Set to Screen" proposes. It seems that the taskbar or navigation bar is subtracted. I also checked the "Show Next Song Indicator" and "Always Show The Title Bar", they don't have any influence.

I also tried setting page size 1860 x 2354 (the screen size of the Boox) on the HP Detachable.
The behaviour is the same. After copying the database, it's -1 / -1, correcting that makes the song look the same.
Trying that I noticed something else:
As soon as I set fixed page size to 1860 x 2354 the font becomes very small (probably as expected, previous font size 35 is kept unchanged). Auto-size font just increases it to 59. That's limited by "Maximum Auto Font Size" = 60. Setting font size manually to 84 increases the lyrics to the desired size.
So I tried setting "Maximum Auto Font Size" = 200. This setting is taken over (it is still displayed after a restart of MobileSheets) but it has no effect: Auto-size font still sets fonts to 59.
           

I'm not sure how to proceed with preparing the move from the HP Detachable, my current 'active' device that I use on stage to the Boox, that should take over in future.
My gut feeling says that I should better set fixed page size to 1860 x 2354 on the HP and adapt the font sizes, then copy the whole database to the Boox. I guess that would make the best use of the much higher screen resolution of the Boox, but I did not investigate that in detail.
If so, we should find out how to solve the issue that the page dimensions are lost. And we should enable auto-size font to increase the font as necessary, so probaly "Maximum Auto Font Size" needs a fix.