I am trying to backup my old iPhone 7 to my Mac, so that I can restore it to the new iPhone XS. iTunes keeps giving me an error stating Unable to backup due to insufficient space on the computer. The iPhone that I am trying to back up is 128 GB capacity (with 70 GB free, and I have 128 GB free on my Mac.
What's the probable problem here? Software on all of my devices are up-to-date (macOS Mojave on my Mac and iOS 12.1 on the iPhones).