Why and When does the MS Outlook 2003 pst file size change when messages are deleted?

I use the Mailbox Cleanup panel in MS Outlook to view my mailbox size, view deleted items size and to empty the deleted items folder.    Say my mailbox is sized at 300 MB and there are 50 MB in the deleted items folder.  When I empty the deleted items folder, the mailbox size reduces to 250 MB.  But when I immediately look at the size of the Outlook.pst file, I will often see a bigger number, somewhere between 250 and 300MB  usually closer to the 300 MB size as if I hardly deleted anything at all.  When I check the Outlook.pst size two or three days later, Outlook.pst will be reduced down to 250MB size.
What is going on?
Why the time delay?
Can I issue a command that will force the size reduction in Outlook.pst after I empty the deleted items folder?
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Hello jsgreiner,

You have to compact the PST file to gain the space due to deletion of emails.  
1. On the File menu, click Data File Management.
2. Click to select your Personal Folder, and then click Settings.
3. On the General tab, click Compact Now.
4. Click OK, and then click Close.

Hope this helps!
jsgreinerAuthor Commented:
I understand that I have to compact, but does that happen automatically after a certain amount of time?  why would it appear that I got the space back without having to manually compact?
Automatic compaction takes place as an idle task in the background. The following conditions must be true for this background task to take place:
"      You must be running Outlook.
"      Your computer must not be engaged in other CPU-intensive tasks such as copying or downloading files.

Reference: http://support.microsoft.com/kb/289987

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