Have you checked to see if the memorystick is a fake? One way to tell is the press Triangle whilst on the memory icon and to go to information. If it says Magicate Supported then your stick is legit, if it says unknown then it's likely a fake.  Fake sticks tend to corrupt data and when formated will end up being smaller than it was originally! If it's legit then I don't know, I've never experienced anything like that before so I can only assume it's fake.