SharePoint/SQL backups taken with AOFO (Advanced Open File Option) are considerably larger than regular backups.
SQL databases are physically larger than the data held within them. This allows it to expand if required when more data is added. When the backup is done using the SQL agent it will backup the SQL data itself, not the physical files, so any empty space will not be included.
When the Backup is done using AOFO and SQL Agent the Backup Job will backup and show the physical file size of the SQL Database.
Symantec recommends that SQL backup jobs be run separately from AOFO or ADBO backup jobs because SQL backups made with snapshot technology are considerably larger than regular SQL backups.