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The only way to reclaim disk space in a Confluence system is to reduce the amount of content you have. You shouldn't be thinking about the database size, all that is doing is trapping your data, individual table sizes have no relevance because most people don't even know what is in them, let alone actually care.
To clean up the DB and reclaim space, without crippling or destroying your Confluence, do some housekeeping - delete old pages, spaces, and attachments that are no longer useful.
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