Why isn't my Outlook PST getting smaller after deleting emails?

Much like Windows itself, your PST has a an equivalent to the Recycle Bin called the "Deleted Items" folder. By default, when you delete an email message (or just about anything that Outlook stores in your PST), it's not deleted at all, but simply moved to the Deleted Items folder.

The whole point of Deleted Items is as a safety net; a way to recover from "oops, I didn't mean to delete that" because things aren't really deleted.

So if things aren't really deleted, then of course no space will be freed up.

You can configure Outlook to empty the recycle bin when you exit Outlook, or you can simply empty it yourself. In the folder list, right click on the Deleted Items folder and click on Empty "Deleted Items" Folder:

This will permanently delete the items from your PST.

However your PST won't get smaller. At least, not right away.

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When Outlook actually deletes an email (i.e. when it does not put it in the Deleted Items folder, or when it removes it from that folder), the space taken up by that email is "freed". In other words that space within the PST is marked as unused, and made available for other email messages that might arrive thereafter. So your PST won't get smaller right away, but it's also quite likely that it won't get bigger as new mail arrives either. The space freed within the PST by emptying the recycle bin will simply get reused.

I keep saying "right away" because Outlook will start making your PST smaller, after some time.

If you leave Outlook running, after some amount of idle time, it will start to perform something called "compaction". Compaction is very similar to defragmenting your hard disk. During compaction it moves all of the email messages and other "real" information in the PST to the front of the PST, and moves all the unused or free space within the PST to the back. When compaction is done, Outlook can then actually make the PST file smaller by removing that portion of the PST that isn't actually being used by anything.

That's where patience comes in. If you empty your Deleted Items folder, and just leave Outlook alone for a while, sometimes a long while, it will dutifully compact and shrink your PST.

If you're impatient, like I am, you can force compaction.

In folder view, right click on Personal Folders:

And click on Properties for "Personal Folders" .... You'll get this dialog:

Click on the Advanced... button, and you'll get this:

And as you might guess, hit that Compact Now button. You'll then see this small dialog for a while:

Exactly how long will depend on the size of your PST, and how much free space there is within it. It can take quite a while for larger files.

When it's done, your PST will be as small as it can be to hold what's inside.