Untill Oracle 9i version you could reduce the size of backups backing up only used blocks and skipping unused blocks: in this way you would reduce only the backup sizes of datafiles that were oversized or had significant free space.
Oracle 10g RMAN has introduced the capability to compress backups: indeed it is possible now to compress backups regardless of the contents of the datafiles, using the new BACKUP AS COMPRESSED command.
Compressed backups work only with backup sets (database, tablespace, and datafile backup sets), so you cannot compress image copies.
The following command will backup an entire database using a compressed backup
RMAN> backup as compressed backupset database;To configure the default backup type for a compressed backup set use the
RMAN> configure device type disk backup type to compressed backupset;Compressed database backup sets are compressed at approximately a 5-to-1
ratio, or 20 percent of the size of a standard backup set.