I've had similar problems. Not sure this is the best method, but it worked for me and I don't think it can hurt.

If you're able to open Outlook after running scanpst, do these steps BEFORE you exit:

1. Check for messages that have large attachments. If you still need the attachments, save them outside of Outlook and then delete the messages in Outlook.

I did this in my Inbox and Sent messages folder.

2. Delete as many other messages as you dare. (I deleted thousands.)

3. Delete as many messages from the Deleted folder as you can. Maybe all of them.

4. Right-click your mailbox. Choose "Data File Properties" from the context menu. Click "Advanced", and then click "Compact Now."

If you deleted many messages during the above steps, compaction will take a long time. This is the only way to shrink a bloated pst file.

5. When compaction is done, exit Outlook.

Restart Outlook, and hopefully you're good to go!