My largest folder under sync is 2.72 TB with 574,531 items on external USB disk: Mac os x Extended (Journel, Encrypted).
That’s 9.05 KB per file. Seems kinda high. Does this index use like 4096 keys and stuff? That would explain it I guess a key that big would be 4kb etc.
Count on something like 48 bytes per 128 KiB block of data managed, plus maybe 80 bytes or so per file, for each device, plus some overhead (database slack prior to compaction etc) and indexes.