Restore only blocks changed after last backup

Hi for some reason I have got an idea:
With Proxmox Backup Server working with PVE server I can restore VM as only blocks changed after last backup are transferred over lan.

But testing restore VM always copy all actual used blocks in vm disk
vm having 40GB disk (tested now with directory xfs/qcow2 and VLM thin. just LVM not tested yet).
There is about 15GB actual windows data.

1. firts backup: about 15GB data transferred a
2. second snapshot backup: few megabyte data transferred

3...

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