This could be encountered in NetBackup versions 7.1 and higher where Automated Image Replication (AIR) is used. All versions of PureDisk 6.6 and higher when used in a PDDO (PureDisk Deduplication Option) configuration may also be affected.
A new Technical Article has been published today that discusses a number of possible causes and solutions for the source and target storage pools not being the same size, which has been a cause for concern for some NetBackup customers:
Support Knowledge Engineer
NetBackup Support - Symantec Corporation