If you use Time Machine, exclude virtual machines from the backup.

Any change to a virtual machine's virtual hard disk file results in the entire file being backed up. When you have a 40GB virtual machine, having Time Machine back it up every hour can fill a hard disk quickly.

To prevent problems with your virtual machines and to save space on your backup media, you should not back up virtual machines with Time Machine.

To use Time Machine to back up data and documents from your virtual machines, keep those files on the Mac.