Reindex + growing storage size

[mkovrigovich (Max K)]

Hi,
in our system we do backups using reindex command with wait_for_completion=true like

    POST _reindex/?wait_for_completion=true
    {
      "source": {
        "index": "master_data"
      },
      "dest": {
        "index": "data_backup_timestamp"
      }
    }

then we populate the master table with new data

after that command I checked the backup index and got numer of docs in backup index the same as in the master index, that is ok.

if we run the following command manually a few times

GET _cat/indices/?v

we see the backup index store size is still growing, I wonder why?
The thought is once reindex is completed then no actions are performed. Growing index is confusing to me a bit, so unsure if the backup has completed or still running
Could you please explain that behavior?

Thank you in advance!