BC_ARCHIVE archives archive administration data. The long text that you entered for your archive runs is what is archived from STXH and STXL, but those tables contain the longtext from any application in your SAP system. e.g. from material master, from sales orders, from purchase orders, invoices, deliveries.....
If you want reduce the content of STXH and STXL then you have to do archive runs for all your other data.