While working in Parallels Desktop virtual machine you receive this popup message:
Unused space on the VM_NAME virtual machine hard disk has significantly increased. Do you want to compress the disk? When the compression is over, the space initially occupied by the virtual machine hard disk(s) will be reduced from XX GB to XX GB.
An "unused" space on your virtual machine has increased. Unused space appears when you remove any data from the virtual machine.
In order to compress the virtual hard drive - click Yes.
You will have to PUBLISHED the virtual machine, so it is advised to shut down all the applications first and save all unsaved documents.
Go to Actions -> Configure -> Hardware -> Hard Disk 1 and click the Compress button.
If you don't want to receive these notifications in a future go to Configure -> Options -> Optimisation and disable automatic compression: