I think nothing is wrong with your setup. You have activated the "delete" option in the rsync dialog which is necessary to get the same content for source and destination.
The difference in size may be caused by the different drive structures which are used for source (mdadm raid) and destination (single disk). Is "stor1" an external USB device? Some external USB enclosures do a 512 Byte to 4kB sector emulation when the disk is bigger than 2 TB to be compatible with older OS`s. It´s the overhead which depends on such parameters which causes the difference in size.
In my opinion you can do nothing against it.