Did u use javax.microedition.rms.RecordStore's getSize() functionality to come with the mentioned figures?
r u testing this on the emulator or the device? If you are using emulator try to clear the RMS manually or thro' the option provided in the emulator and then check the readings.
Basically RMS implementation must add in few additional header information to identify the data that is stored but definitely not in the ratio that you had mentioned.
~Mohan