Hi
Surely around more than of 90% if these PO are STO. If you are working with a SAP IS-Retail you need to archive documents periodically. Run for instance tcode TAANA and check what kind of PO documents you have in your database.
Take in accound other tables with a fast growth could be MSEG, LIPS, BSIS and so on.
Check with your FI consultant if it's possible to do summarize documents in FI and CO-PCA.
So, begin to think in Archiving as your next task.
I hope this helps you
Regards
Eduardo