Size of store directory after migration

Good day!

I try to migrate my old zimbra 7 to zimbra 8 OSE but faced with the following problem.
The size of zimbra store directory on my old server - 764GB
Backup size - 538GB
But zimbra/store directory on the new server has 929GB already and it's still increasing.
RealTime scanner was enabled but, as I understand, backup operation wasn't perfromed cause /zimbra/backup/zextras has only 35MB.
Is it usual situation?