The Compression setting "Extreme" brings the smallest files size, if I understood correctly.
But does the "Storage optimization" setting also affect the file size? Or is this only relevant for how much data has to be transferred?
The smallest size will be achieved with Storage optimization = WAN target, Compression level = Extreme. First chooses the size of data block, second how hard backup proxy will try to find similar data blocks.
Other question: Higher compression settings need more CPU usage. But of Which host?
In case of backing up VMs the CPU of the Veeam B&R server and in case of backing up Windows PC via Windos Agent the CPU of the Windows PC? Is my understanding correct?
On backup proxy for VM backups, on PC running Veeam agent for physical servers.
Any other settings which affect file size at final target store?
Hardware deduplication on target repository.
I would suggest to read this