Two thoughts immediately come to mind:
1. If the guest is downloading macOS updates in the background, that will consume additional guest disk space, and some of those updates are massive. I usually leave the network adapter "disconnected" while installing VMs for that reason.
2. If you have FileVault full-disk encryption enabled in your macOS guest (in System Preferences, under Security & Privacy, choose the FileVault tab), the guest will encrypt the contents of the virtual hard disk, including the unused regions of the disk. Fusion will need to write those encrypted disk blocks to the virtual disk file on the host, and this will cause the VM's virtual hard disk to expand out to its maximum size, just the same as if the disk was preallocated. That expansion will occur in the background while macOS performs the encryption.
You can try running:
sudo du -sk /*
inside the VM to get a picture of where the guest is using disk space, but keep in mind that the disk usage shown there (in kilobytes) will not include the macOS Recovery HD and Preboot environment which are in separate APFS containers.
I hope this information is useful!
--
Darius
