However, some of this overhead is offset by the space saved from having to track 16x more clusters when using 4K. In relatively small scale testing (100's of GBs, not TBs) to this point the space difference between 4K and 64K clusters is <10%, although the exact amount will vary based on the selected Veeam block size and compression ratio achieved with the specific data set (the above results were using Veeam defaults and a small mix of Windows and Linux VMs). While I don't know this for sure, my guess is that Microsoft introduced 4K clusters for ReFS to make the filesystem more attractive for generic file server workloads, where the overhead of storing millions of relatively small files in 64K blocks was a huge barrier to acceptance. I was able to format even a 250TB volume with 4K clusters so I'm not exactly sure when the cutoff is where you have to use 64K in Windows 2016. In versions of WIndows prior to Windows 2016 the 64K cluster size was the only available option for ReFS, however, on Windows 2016 with ReFS 3, you can select either 4K or 64K clusters.
