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Why are virtual drives so small?

I'm currently in my trial of Workstation Pro and I noticed that a fully setup Windows 10 Home VM's disk image (one file) is only ~12 GB (GiB) in size before compacting and 9.33 after, according to Windows Explorer.
Ok, it's probably compressed (The VM says 17.1 GB used). Neat I guess... Or so I thought until I compressed the image to only 4.26! GiB with zstd:3.

Why/how is the .vmdk file so small and can be compresssed down by over 50% with a fast and light algorithm like zstd?

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Disk space for .vmdk files is allocated on the VM's guest operating system's need, i.e. whenever the guest OS writes data blocks to previously unallocated storage. One reason why the .vmdk file may be smaller than what the guest OS reports as used disk space, is the fact that a simple file creation within the guest, (e.g. a large pagefile) will not consume host disk space until the guest file is modified.

The compression that you see depends just on the gust OS files, i.e. how well they can be compressed.