Heres an explanation found on Google - hope this helps ?
Outlook Express stores messages as entries within a database. Over time, these databases can grow in size and, as a result, degrade performance and consume a large amount of disk space. The reason is that when a message is deleted within Outlook Express, even if it is removed from your Deleted Items folder and out of your view, it still consumes disk space. So, over time, all the hidden records, which you thought were deleted, slow down performance and take up disk space.
Outlook Express allows you to compact your folders, which essentially removes all the records that have been marked for deletion from your computer. Compacting should improve performance and free up disk space.
By default, Outlook Express is configured to compact folders automatically during periods of inactivity. You can turn off this feature using the Maintenance tab from the Options dialog box. However, should you choose to disable this feature, you should manually compact your folders once a month. You can do so by selecting the Clean Up Now button located on the Maintenance tab and clicking Compact from the Local File Clean Up dialog box.