2. Outlook's attachments make it massively bloated
If you regularly send and receive attachments, your Outlook .pst file can quickly become massively bloated. It's pretty easy for your .pst file to quickly get to 250MB or more, and I've known people whose files range up to 1GB and beyond. Among other problems, this slows down the speed at which Outlook loads and can lead to instability.
It's time to put Outlook on a diet. First, find out where the fat is. Outlook 2007 has a very useful folder called 'Larger Than 100 KB'. Find it underneath Search Folders in your list of Outlook folders. As the name implies, it lists all email messages that are larger than 100KB. By default, they should be listed with the largest files first, but if not, click the Size heading in the folder until you get them listed that way.
Now that you can see the largest emails, start trimming. If you're like me, you'll be surprised how many of the emails with attachments you no longer need; delete those. If you need the attachment, but don't need the accompanying email, save the attachment to disk, then delete the email.
If the opposite is true - you want to save the email but not its attachment - you can save space by either saving the attachment outside of Outlook or deleting it altogether. First, save the attachment to disk. Then open the email, right-click the attachment and choose Remove. The attachment will be deleted from Outlook, but the email itself will remain.
The attachment problem in Outlook is so notorious that a third party has stepped in with a solution that helps you cut down the size of your .pst files by removing attachments. The free Kopf Outlook Attachment Remover saves attachments from Outlook, stores them on your local disk and replaces the attachments with a link to the stored file. You'll be able to open the attachment as you would normally, except that Outlook will grab the file from disk, rather than from inside its .pst file.
You can have the program automatically go through entire directories, removing attachments and replacing them with links, or you can instead do it email by email. Note that in Outlook, it will look as if the file is still there - you'll see the file icon as you normally do for an attachment. But the file is actually on disk, not in Outlook.
Outlook 2007 includes another tool for shrinking the size of your .pst file by targeting your fattest folders. Select 'Tools', then 'Mailbox Cleanup' and click the 'View Mailbox Size' button. There will be a list of folders, along with the total size of each folder. That will tell you where you'll be able to get the most reduction by cleaning out emails - and their bloat-inducing attachments.
After you find the largest folders, go back to the Outlook main screen and tackle those first, searching for unnecessary emails and attachments and deleting them.
After you've used all of these techniques for deleting attachments and emails, it's time to compact your .pst file. Normally, when you delete files and attachments, there are essentially blank spaces left in your .pst file that take up bytes.
Compacting the file eliminates those blank spaces and shrinks the total size of the file. To do so, select 'File' and then 'Data File Management' and double-click the 'Personal Folders' entry. Then click 'Compact Now'.