In order to convert your VHD or VHDX file from a dynamically growing format to a fixed size format,
you can use the PowerShell and run the Convert-VHD command.
When the VM is offline, you can specify the VHDType parameter and switch to a differencing disk, you can switch between Fixed, Dynamic, and Differencing as needed.
The syntax to convert to a dynamic disk type is as follows:
Convert-VHD –Path c:\VM\my-vhdx.vhdx –DestinationPath c:\New-VM\new-vhdx.vhdx –VHDType Dynamic
More information on this command may be obtained here
Also, try BackupChain http://backupchain.com/Backup-Software-Protection.html to back up your VMs quickly and efficiently