"Sent Items" is one of the Default Folders OE creates when first installed. (Along with Outbox, Deleted Items, Drafts and Inbox). It doesn't actually appear until the first e-mail is sent.
What you see as a "Folder" is actually a File named "Sent.dbx" and it resides here:
C: \Documents and Settings\fobotcso\Local Settings\Application Data\Identities\{1A2CD489-EC15-433F-A2B6-EDD9B78272BA}\Microsoft\Outlook Express
You could see if there are now two similar files and which one is growing as you send e-mails. Maybe there is a "Sent itmes (1)" that is only about 75KB in length abd static but you are seeing it in OE as "Sent Items". Try deleteing it.
But first I would see if, from within OE, I could delete the worthless Folder called "Sent Items" and re-create a new one.
You will understand if I don't try it here first, I hope!
All that's happened is a Registry glitch and it could be sorted by editing the Registry but this needs care and planning - and only after a full back-up of all the files in the above Folder.