DVD Decrypter is not supposed to compress movies. It does exactly what it says. It rips and decrypts dvds for you (along with the bonus of burning image files too). If you want to compress dvd9's then stick with dvdshrink, dvd2one or Instantcopy (the best 1 clicker out there).
If you read the Readme file that is on the DVD Decrypter site you would know this
