Originally posted by marist89

However, since each database block is not 100% full with data, I am thinking that the 20% difference comes from having to read more overhead in the smaller blocks. I would like to find some documentation that supports or disproves my theory...
You are right about the block overhead and thats what I had pointed out. Smaller the block size, more blocks used and more time for backup. Secondly it doesn't matter if the block is 100% used or not. Even if the block is empty during the backup and was used by Oracle previously it will be backed up.