Cleaned up 13 gigs of mystery data... (18 with cubasis too...)
In iOS settings my BM3 size was being reported as 75.56 gigs. I copied all of the contents to my PC (twice, once with iTunes and once with iMazing) and both copies came in at 58 gigs. Anyway, I deleted BM3 from my iPad and copied everything back to the iPad from PC. Now it is reporting back as 62.66 gigs. So about 13 gigs saved. Not sure what the overhead was but I imagine 'Files App' temp files or some such thing.