So from what I could observe between my 920 and now my 1020, and read from others all around, it seems like the 1020 has always over 2 gigs of other Storage, while the 9xx only have bit over 1 gig... The only difference besides the camera between these is the RAM.
The 9xx have 1 GB and the 1020 has 2 GB. I think the page file (which is often automatically set in Windows to be the same size to 1.5x as the physical RAM) is included there, too and therefore it will never get smaller than that. But at least it shouldn't grow much further.