I just did a major archiving run using Outlook's "Include items with "Do not auto archive" checked.  This generated over 1 GB of archives that had not been archived before.  But when I checked the Outlook.PST file after this, it had actually grown from just over 3.x GB to over 4 GB. 

Why didn't the Outlook.PST file shrink in size to match the amount of the new Archive.PST file I created just for this big archive run?