This may be a normal occurrence if database grooming of the Information Store has recently been performed. After database grooming in Exchange, a backup job may still take up the same amount of Disk or Tape space even if the actual size of the Information Store in question is smaller. The backup completes successfully with no errors and the data can be restored, the only issue is that the backup job takes up more space than expected.
If the Exchange Information Store is groomed to reduce its size on the Exchange server or within the DAG and the passive copy is selected for backup, Exchange can fail to replicate the logs and in turn report the Information Store as larger than it actually is. If this happens, the backup set will continue to take up the same amount of backup media even if the actual size of the data being protected has decreased.
In order to have Exchange report the correct size to BE and thus only use the amount of media actually needed:
1. Create a separate job to backup only the Information Store or DAG in question.
2. This job should be a full backup of the Active copy of the Database. It should truncate logs.
3. Ensure that GRT is not enabled and do not set up any Differential or Incremental jobs to run afterwards.
4. Run this job as a one time Instance.
5. After this job runs to completion, go back to the original job(s) and select the Full Backup and Run Now.
Compare the size of the Backup set to the size of the Data being protected and it should now be correct.
BE 2010 R3 backing up Exchange where BE is performing a backup directed at the passive copy of the Information store and the Active copy of the Information Store has been groomed in such a manner to decrease it's physical size in the Exchange server or DAG environment.