Can not change private storage size

I want to reduce the size of private storage in several VMs, but it just ignores my change. I checked if its enough space there, but still can not reduce the space.
Why that?

Best regards

#Edit: it’s because of possible damage on the data, isn’t it? So is there no safe way to reduce the private storage size?

Assuming default LVM Config

Unused storage in a private volume is eventually return to the pool as free space regardless of the size of the private volume. You’re only using what’s allocated within the private volume, not the entire entire space.