Well that was nice! I was having this problem where I couldn't get the latest update to OSX Mojave installed on my laptop because I didn't have the space. I've spent a lot of time eyeing iPhoto Library.migratedphotolibrary which seemed like it was taking up 52GBs of space, but in theory that is just duplicate spaced used by the new Photo Library with both files hard linking the same data on disk.
I finally took the steps to copy the migratedphotolibrary off the internal hard drive to the Drobo and the delete the the bundle and... yep, I got 52GBs of free space back. So it looks to me like some time during the migration from laptop to laptop some copy operation changed those hard links into duplicate files. At least I've got plenty of space now, right?
Update: It was pointing out to me the most likely culprit was the conversion to APFS.