Hello Ganimede,
Is it possible to perform a system copy (note 151603) after the archive project? If yes, you could upgrade your SQL Server release to 2008 (or even 2012) if your system has the required SP stack (check PAM).
Using SQL Server 2008 (or 2012) you will be able to use the compression (note 1488135). Also check the following blog:
DB Size decreased by 81%? Awesome
Regards,
Eduardo Rezende