Why RAW-Preallocated disk is converted to RAW-sparse after doing cloning/exporting/importing/disk migration of VM

Issue

• The disk was automatically converted to RAW sparse (Virtual Size =/= Actual Size ) from RAW preallocated (Actual size == Virtual size) after cloning a VM.
• Why the actual size of the disk got reduced after migrating the disk of a VM from one storage domain to another?
• Actual size of the disk is reduced after importing the VM from the export domain.