The way these drives are faked to show higher capacity is by modifying/hacking the firmware, so the drive reports more storage then there actually is. I think you would need to somehow revert that firmware change. But keep in mind that these drives are notorious for losing and corrupting your data once you exceed its actual storage limit. The fact that you can write to the drive does not necessarily mean you can read the data back.
That being said, a quick google turned up this page which outlines a procedure to flash corrected firmware using MPTools. I have never used it so use at your own risk: http://www.myblog.bloggybloggy.com/usb-key-fix-mptools-11-05-2008/
Edit: Also, read these:
Unless the pen drive contains one of the popular controller chips
(Microv, ICreate or Alcor families) and memory storage chips (Samsung
& Hynix) finding the correct low level software program will be a
challenge. You will spend a significant amount of time looking for
solutions and may not be successful in your efforts.
You may find some software on a website that you think could fix your
drive and inadvertently download a virus or other forms of malware.
McAfee’s Siteminder identifies some of the download sites as
containing malicious software or software that breaches browser
The drive may also be irreparably damaged during the repair process.
Using the wrong software can destroy the flash storage chip. Using a
program someone else used with their previous sitting i.e. leaving the
ECC open could destroy access to the storage chip, as information in
the controller is over written.
Could you ever trust a repaired drive with your data files or
pictures? The unscrupulous creators of the fake flash drives maximise
their profits by using the lowest cost chips they can purchase. The
quality of these chips range from average to poor. When the chips fail
you may be lucky and just loose all the files that you have stored on
the drive, or worse – the contents of the files can be corrupted and
remain undetected by you.