There are a couple of commands you might use, but I don't kow if they are going to give you what you want
omnirpt -report session_statistics -timeframe [start] [finish] -tab -log [path to filename]
[start] and [finish] give you a timeframe to look at and must be stated in this format
yyyy/mm/dd hh:mm
-tab will output a tab-seperated file which you an import into Excel
-log [path to filename] gives you the opportunity to state a destination path
So, for example, to get a summary of all backups from Jan 1, 2012 to yesterday, the command would be
omnirpt -report session_statistics -timeframe 2012/01/01 00:01 2014/06/30 23:59 -tab -log D:\stats.txt
Now, I don't know if this is raw data showing everything that has been backed up, or just backups that are still under protection
The other command is
omnirpt -report media_list -tab -log [path to filename]
In terms of trying to free up some space, see what sessions are the oldest, and either right-click on the media, pick Change Data Protection, and set it to None, or Recycle the Media used for that session. Every day, DP looks for sessions that have no protection, removes teh catalog from teh IDB, and removes the media if it is File-type
