Zimbra's default configuration will delete backups older than 1 month old. You can modify this entry in cron. Here is a snippet to delete backups by hand from the wiki..
Delete backup sessions either by label or by date. Deleting by label deletes that session and all backup sessions before that session. Deleting by date deletes all backup session prior to the specified date.
zmbackup -del 7d
More info can be found here:
http://wiki.zimbra.com/index.php?tit...ckup_Procedure