Synching two NAS's with different sized Drives

I have a new DS923+ with two 9tb drives. I am hoping to use my older 718+ at a second location and synch the NAS’s. Will I be able to do this with there being smaller drives on the older unit (4 tb) or will I have to add 9 tb drives to the second unit? I have less than 4 tb of storage used on 923+.

When syncing, the size of the drives is irrelevant. What counts is whether the storage space on the destination NAS is sufficient.