Almost all dvd movies are dual layered(dvd9) except for some early dvds. Blank media only holds 4.7GB. There are two options, either take out extras, unused audio streams, subtitles, etc. If you take out everything but the movie and it's still too big then you need to encode the video to a lower bitrate. All this takes alot of work and time. The other option is much easier. All you do is split the movie onto 2 discs. You get to keep everything that way. You can read the tutorials on how to do this at
doom9.net.