Thanks a lot to pointing to these facts! I feel reassured again somehow ;). Weird... then this might really be some new weird bug in Windows 10. I know that it is showing these big blocksizes since at least may, where I discovered it because I wanted to know how large a backup of my library could get. And I almost couldn't believe this discrepancy.
Is the SMB.conf of v7.90.008 ALPHA for C2 the same as on your RPi and NUC then?
have a look at this: Linux Performance - SambaWiki
The third paragraph (from this URLs position) says that 1MB is default.
...if the allocation size is set to 1MB (the default in Samba) then...
I don't really understand that. Do the other systems use a different samba version then, so that your systems show the correct size?