Backup datafiles size in RMAN

[goodhealth]

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> }

[mbhima]

Since there are more transactions in db2, fragmentation or empty blocks in between the data blocks may be the reason for different file sizes. If you take block level incremental backups, then you may get the same size backup files.
Check for fragmentation or for empty blocks. See how many blocks a table is taking in both the databases.