With XP Service Pack 2 Microsoft made a number of changes to how Outlook Express compacts your emails.

As with any storage in Windows (mail, database etc) when you delete data it is only marked as deleted and not fully removed. Running compacting tools will delete this data and free up space.

In Outlook Express the compacting starts after you have opened and closed Outlook Express 100 times. Unfortunately this value cannot be increased or decreased within the software. However, there is a registry hack to reset the counter. I would warn you though that changing the registry can cause severe problems if not done correctly.

1. Close Outlook Express and all message windows.
2. Click Start, click Run and type without quotes "regedit".
3. Navigate to this key
   
   HKEY_CURRENT_USER\Identities\{GUID}\Software\Microsoft\Outlook Express\5.0
   
   where {GUID} is the long number used for your Identity.
4. In the right-hand pane, double-click "Compact Check Count", then change the value to zero.

Alternatively you can install a tool which will compact your mail and reset the counter. Unfortunately I have not tested this tool and cannot offer support on it but to try it then please visit:

http://www.oehelp.com/OETool/Default.aspx