There is an upgrade to a new appliance/VM that is a different version than the current Symantec Messaging Gateway (SMG) currently in production use.
The error "file too big" may occur if trying to restore a database file to a version that the backup was not created on.
It is not supported to attempt to restore a database backup on a version of SMG other than the version that was used to create the backup. If attempted it is expected that the operation will fail.
Database backups can only be restored on the version that made the backup (i.e. 10.5.1 backups on work on 10.5.1 SMG).