|Are you meaning the "dbx" file called Sent Items?|
If so, I believe the only way to reduce its size is to delete it..
Create a new folder in Outlook Express (call it something like Temp Sent and move all messages from Sent Items to Temp Sent. Close Outlook Express. Delete Sent Items.dbx. Re-open Outlook Express. Transfer the messages back into Sent Items. delete Temp Sent.
The recreated dbx file should now be much smaller.
(Same applies to Inbox, Deleted Items, etc.)