Abnormal disk space usage on macOS 10.11 disk image

As you can see in the picture, macOS 10.11 tells that I has used around 16GB of disk space. However, in host macOS, it shows the disk image is 37.5GB, which is more than double of the disk space usage in macOS 10.11 really using.

I don't have any snapshot either. So where is the missing space and how can I fix this?

 

According to Disk Utility in Snow Leopard Server, I'm using 13+ GB of my VM's HD. Yet the actual size of the .pvm in Mavericks is 26+ GB. It must be the way VMs work - in other words, instead of allocating free space dynamically as you need it, it recognises that any incarnation of OS X needs to have that free space already in existence in order to function properly. That's my guess anyway.

 

As you can see in the picture, I have many instances of macOS, from 10.10 to 10.13. All the others are not that big. 10.13 disk usage is big because I have installed Xcode 9.

I believe it is a bug of Parallels Desktop. In normal situation, the disk space should be bigger than the real usage of guest OS, but not twice more.

 

Problem solved.

1. Upgrade to the latest version on each of the macOS you used.
2. After the upgrading and re-login to macOS, shutdown the OS completely.
3. Choose the setting of virtual machine and reclaim the disk space.
4. If the reclaim is gray, power on the virtual machine, login and then shutdown it again.
5. Go to 3.

Here is the disk space usage after.