Creating an annual archive

I want to limit the backup file size on my computer. At the end of each year, I want to move the backup file to a DVD data disk and then start with an empty backup file, backing up only data from the beginning of the current year. I think this would be easy, if I could just find the backup file. I initially directed Gmail Backup to put the backup file in the folder:
c:Program Files (x86)\GmailBackup\GmailBackupFile
Now, even though my backup file must be several hundred MB large, the GmailBackup folder size is only 18MB. Where is the backup data file?