My 64G iPhone started complaining that it was running low on memory. I thought this was unlikely, since I didn't have much more on it than I used to keep on my 16G iPhone.
So, I started deleting pix and music and anything else that looked large. But, I couldn't get the memory usage past 5G free.
As a last resort, I did a backup / reset / restore. After the restore and putting all the music back, I now have 40GB free. So, that's great. I wish I knew what was taking up all the memory, though.