After archiving about half the messages in Microsoft Outlook 2003, the main PST file has not changed in size. Isn't the point of archiving to keep the size of the PST file under control?
Try compacting it.
I believe the .pst format is a lot like databases in which removed records are "tombstoned" instead of physically deleted. This is done because it would slow things down a lot to rewrit the entire .pst file every time an email was deleted.
So the spaces where your archived email did exist in the old .pst are marked empty and will be used to hold new data, but those spaces weren't removed. Compacting removes the empty space.
Reduce the size of a Personal Folders file (.pst) If you delete items from a Personal Folders file (.pst), the file is automatically compacted in the background when you’re not using your computer and Outlook is running. If you want to immediately reduce the size of the file, you can force a compaction. The procedure to compact the data file might take several minutes.
On the File menu, click Data File Management. Click the data file that you want to compact, and then click Settings. Click Compact Now. NOTE You do not have to exit Outlook after you compact a .pst file.
I have encountered exactly the same issue in Outlook 2007 - however, I found that compacting didn't help. The first time I compacted the PST after archiving, it ran for about 20 minutes, but the PST size remained at 2.7GB. I compacted again and this time it finished in seconds, suggesting that the first run had actually done something, but the PST size in Windows Explorer remained the same.
I then tried exporting everything in the Outlook folder to a new PST which then showed in Windows Explorer as 50MB.
I've now gone through this procedure on several computers and it seems that the only way to reduce the actual PST file size is to Archive and then Export.