If you’ve used a Windows virtual machine under VMware (Workstation or Fusion, depending on your host OS), then you’d have probably noticed that the host virtualisation software cleans up the virtual machine disk once the virtual machine is shutdown. The clean up activity detects any unused space and reduces the sizes of the virtual machine disk within the host OS without affecting the guest OS.
I have a number of Linux VMs but noticed that there was an increasing discrepency between the size of the used space form within the guest OS (via df -h) and the space consumed by the virtual machine disk. The reason? VMware doesn’t automatically run the clean up task once a virtual machine is shut down.