Outlook.pst file remains same size even after deleting emails

My Outlook 2007 outlook.pst file, under Vista Business, is some 2.7gb. I wanted to reduce it, so i exported all emails sent and received up to the beginning of this year. I then deleted the emails that i had exported from quite a few, well-oragbnised folders. outlook.pst remained the same size. I move all the exported files to another directory. I’m not using exchange myself. Explanation and cure?