.edb/.ost file size/archive issues

The exchange server is running NT server. The client boxes are running Win98 and Win2000. We have users who access exchange remotely and use .ost files. In order to reduce the .edb files on the server, I have had the users archive all of their messages over 30 days old to a local archive.pst on their machines. This has not seemed to help reduce the size of the .edb files. Any ideas on what we might be doing wrong?