a directors commentary is usually just a 2nd audio stream. you’ll find that most ( s 90+%) of the original DVD space is taken up by the video stream. the movie video will only be recorded once onto the DVD, when you choose to watch it with directorys commentary it just gets played with a different audio stream.
So there will be 2 or more audio streams + some menus & maybe some extras. one of the audio streams will be the directors commentary.
you may have to preview the DVD with another program like powerdvd in order to work out which audio stream is which.
you can then tell dvdfab , in customise or main movie mode, which audio strreams to copy & which to leave out, but you don’t save a lot of space that way - you’ll still probably need to compress the movie