How was Commvault used to migrate the data, was it orchestrating Snapmirror in the background, or was Commvault the actual data mover. Is it possible that Commvault either compressed or worse, encrypted the data, making it non-compressible to ONTAP?
The instructions for running post-process sooner are in that KB pointed out above. Run a 0-day cold data scan:
volume efficiency inactive-data-compression start -vserver <vserver> -volume <volume> -inactive-days 0