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_at_$RMAN_CATDB;

backup as compressed backupset database; EOF And

2)
$ORACLE_HOME/bin/rman <<EOF

connect target;
connect catalog $RMANID/$RMAN_at_$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.

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