doing the  hotbackup on the hidden node, and its dataPath is increasing, far lager than the primary and Sencondary node

We do the  hotbackup on the hidden node, and its dataPath is increasing, far lager than the primary and Sencondary node  during hotbackup. 

    s->open_cursor(s, "backup:", NULL, NULL, &c);

   while( (ret = c->next(c)) == 0 && (ret = c->get_key(c, &filename)) 

   {

       copy files to slow disks

    }

    close_cursor(c )

these files becomes larger during hotbackup:

     journal files,

     collection files,

      index files .

we restart the hidden node when hotbackup ended, journal and files collection files can decrease to the size  the primary,  but the index files still so large 

the process of hotbackup time is very long (sometimes about sevral hours to more than 20 hours) , because size of the data is large and we backup to very slow disks

 some instances  abort the disk of the datapath:
Primary                    135G 
Secondary                133G
Hidden                      334G
------
Primary                    211G 
Secondary                213G
Hidden                      330G
------
Primary                    183G 
Secondary                194G
Hidden                     416G
------
Primary                  964G 
Secondary              964G 
Hidden                    1.5T