I've come across this problem a few times now.
- I initiate a transfer of an iMovie project from my iPhone to my son's iPad (both with current software as of August 2016: iOS 9 and iMovie 2).
- The transfer appears to complete, but then we see Insufficient Space. There's not enough disk space to copy this project. About X MB more space is needed on my son's iPad.
- We free up some space on my son's iPad (more than X MB) and repeat the transfer.
- Sadly, we see the same error message. The free space we created in the previous step has disappeared.
- We free up some more space and repeat this process, my hair gradually turning white.
Where is the free space going, and how can we successfully transfer the project?