Full WSB backup on Server 2012 R2 with VMs - incremental?

This is regarding Windows Server 2012 R2 built-in backup feature (WSB),
where this server has multiple VMs installed.

With a full backup, incremental, does WSB perform incrementally inside the VMs?

Meaning, if the vhdx file of a VM has changed, does WSB - when acting incrementally - backup the whole vhdx again, or maybe it can backup only the files (inside the vhdx) that changed?...

(again, this is about a full backup, not a custom one)
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David Johnson, CD, MVPOwnerCommented:
With a full backup, incremental, does WSB perform incrementally inside the VMs?
no it will backup the full .vhdx files of the VM's.. remember as far as the O/S is concerned these are just files and like any other file that is changed it backs up the changed files
campinamAuthor Commented:
Then how can one explain the following:

1. Perform a full backup (the first backup, start with empty USB backup drive).
2. Make some changes to files inside the VMs.
3. Perform a second backup of the host (full, incremental).
4. The total used space on the USB backup drive has increased by only about 2%, while the combined VMs' data is about 70% of the initial backup data.
5. If the .vhdx files were backed up again as full files, one would expect an at least 70% increase in used space on the backup drive (not 2%).

note:  VMs' OS is Server 2012 R2 as well
What you have read is correct
Tried to find documentation on web, but did not found anything proper, hence tested scenario in lab on 2012 R2
The findings:
You cannot define incremental backup for VM level backup, when you fire 1st backup of VM through WSB, it is full backup
Now if you fire same VM backup again, the backup will just take snapshot of changes since last full backup (Incremental) and append it to original full VHDX file
After completion of backups if you look at the backup history details all backups will be listed as Full backup only
However if you are going to restore backup, you will presented all versions and you can restore the version you want (either full OR incremental)

If you taking file \ folder backup you can have advanced options (Configure Performance settings) where you can define incremental \ full backup on drive level.

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