Couchbase bucket size different in across DC

Hi ,
I am using couchbase community version 5.1 with bi-directional XDCR replication DC1 and DC2. I flush the bucket in DC1 but not in DC2 and start the replication. After a few days, I have found out that the bucket which I have flush earlier in DC1 is double in size compare to bucket in DC2.

any idea what can be the issue.