I'm experiencing the same problem... I upgraded from a 128gb iPhone XR with 105gb free to the new 128gb iPhone 12 Pro. I've attempted to do a device to device transfer, iCloud transfer, and a iTunes backup (old phone) and restore (on new phone). All resulted in all my free space being taken up on a my new phone. I literally went from over 100gigs free on my old phone to under 2gigs free on my new phone without adding any new content. Verizon wasn't able to help nor was Apple support. Apple told me to wait 48 hours to see if it resolves. It hasn't. Does anyone have any suggestions?