Backups are just for small installations of Confluence. Once the database starts growing, the database backup tools must be used.

Often, administrators forget about this - they start with a small installation, then adoption grows, and in the end the backups are huge, slow and possibly even broken.

Like discussed over here: http://extranet.atlassian.com/display/SUPPORT/2007/10/30/Confluence+site+backup+is+not+for+big+sites, we should send warnings to administrators once a certain size is reached, or once a time limit is exceeded.