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.