Oracle 11, incremental backup goes wrong
You are doing a differential incremental backup rather than cumulative, but the syntax for recover of copy tag 'sometag' indicates that you plan to incrementally update an image file backup.  Phil was on the right tack with the missing RECOVER COPY OF DATABASE WITH TAG 'sometag'.  Since you are not running this each incremental must be able to recover the un-updated tag 'sometag' causing it to grow each day as though CUMULATIVE were specified. 

See Backup Sets and Image Copies in the Concepts Guide and see  Incrementally Updating Backups in the Backup and Recovery User's Guide (11.2).  If your intention was to use incrementally updated image copies you will want something more like this:

  RUN
  {
    RECOVER COPY OF DATABASE 
      WITH TAG 'incr_update';
    BACKUP 
      INCREMENTAL LEVEL 1
      FOR RECOVER OF COPY WITH TAG 'incr_update'
      DATABASE;
  }


If you didn't intend to do this, then simply remove the for recover... syntax.