RMAN does not do a compress similar to the popular file compression utilities. However, RMAN backups can be much smaller depending on the type of backup you perform. If you perform a file copy type of backup, then the backup will be the same size. It just copied the file. If you do other types of backups, then RMAN will not backup any blocks that are unused. If your RMAN backup size is 1.1 GB and the datafile is 21 GB, then I would suggest that approximatly 20GB has been allocated for the database, but 20 GB contains no data. RMAN's "compression" algorithm is to just skip the empty blocks.
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