So the disk space usage will be provisioned sized + actual size, At the beginning of the task there is a check for space needed eg:
1/10/2017 6:39:33 PM  Backup space needed: 1,928,440,330,570 bytes 1/10/2017 6:39:34 PM  Available space: 2,065,255,366,656 bytes
That doesn't take into account the disk conversion and it might fail with:
 1/11/2017 12:27:37 AM [ERROR]  Error converting disk '/backup/target/vm/2017-01-10-183911/test.vmdk' as thin, (Failed to clone disk: There is not enough space on the file system for the selected operation (13).)
If the conversion is succesful the thick disk is then removed using "vmkfstool -U" command and the thin one is renamed.