I have 2 identical databases, about 39G in size, that we perform cold backup to disk using RMAN. I notice the backup file size for db1 is about 12G and db2 is 23 G. I don't understand why they are so different, only the fact that db2 has more transaction activities. Here is my cold backup script.

8> run {
49> # Cold database level 0 backup
50> allocate channel d1 type disk;
51> allocate channel d2 type disk;
52> allocate channel d3 type disk;
53> allocate channel d4 type disk;
54> backup
55> incremental level 0
56> tag cold_db_bk_level0
57> filesperset 5
58> # Recommended format
59> format '/backup/cold/PSTEST8/bk_%s_%p_%t'
60> (database);
61>
62> # now that the backup is complete, open the db.
63> #sql 'alter database open';
64> }