RMAN Sizing - Anomaly?

Friends --

I'm at a loss to explain why two nearly identical scripts -- one to
backup the database only and one to backup the database plus archivelog
should have this particular sizing anomaly.  First, here are the
scripts... 

1)
$ORACLE_HOME/bin/rman <<EOF  
connect target;
connect catalog $RMANID/$RMAN@$RMAN_CATDB;

backup as compressed backupset database;
EOF

And 

2)
$ORACLE_HOME/bin/rman <<EOF  
connect target;
connect catalog $RMANID/$RMAN@$RMAN_CATDB;

backup as compressed backupset database plus archivelog;
EOF

Now the results...
Database only: 48 minutes - 39.1GB
Database plus archivelog: 63 minutes - 15.2GB

Why should the database plus archivelog be 40% smaller than the database
without?  I've run this test a bunch of different times, sometimes
setting the incremental level to 0, sometimes not; sometimes running the
db+arch first, sometimes second.  But consistently, the db+arch is
appreciably smaller than the db without.  How come?

Thanks!
Bambi.
--
http://www.freelists.org/webpage/oracle-l

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