Rman controlfile and rotation
http://docs.oracle.com/cd/B19306_01/backup.102/b14191/rcmconc1.htm#BRADV111


If CONFIGURE CONTROLFILE AUTOBACKUP is ON, then RMAN automatically
backs up the control file and the current server parameter file (if
used to start up the database) in one of two circumstances: when a
successful backup must be recorded in the RMAN repository, and when a
structural change to the database affects the contents of the control
file which therefore must be backed up.
This means that the control file is backed up in the following
situations:

After every BACKUP command issued at the RMAN prompt.
At the end of a RUN block, if the last command in the block was BACKUP.
Whenever a BACKUP command within a RUN block is followed by a command that is not BACKUP.


In your case, the backup is created before RELEASE CHANNEL. The autobackup would happen after COPY, BACKUP ... ARCHIVELOG and BACKUP ... CONTROLFILE, but since these are in the same RUN block, why perform the work of creating the autobackup 3 times unnecessarily, when it is enough to perform the autobackup after the last BACKUP command?
The size is different because the autobackup contains the SPFILE also.
The autobackup finishes after the controlfile backup, so the autobackup contains the entry pointing to the previous controlfile backup, but the previous controlfile backup doesn't know about the autobackup, that is another difference between them.
When the controlfile grows, it gets more space allocated than it immediately needs, leaving some empty space in it that can be used later. Because of this, the controlfile and the controlfile autobackup can be the same size (when no SPFILE in use), but their content will be still different. Outdated entries can also be reused in the controlfile to prevent the controlfile growing too big.

COPY overwrites the copies of datafiles. DELETE NOPROMPT BACKUP OF ARCHIVELOG UNTIL TIME='sysdate-6' deletes archivelog backups older than 6 days.