Not that I like buying pirated stuff... but recently I got my hands on a DVD with 4 movies in it, and I was really surprised to find that the DVD contained full blown movies with subtitles and the total size was more than 7 GB. How could anyone do it?
This can be accomplished by using a dual layer DVD. Also, if the DVD had 4 full blown movies on it, chances are the quality of the video and/or sound is suffering or the originals had many features (bonus features, other languages) that have been removed.
Transcoding, or more precisely, transrating, that is, reducing the overall bitrate of the video and audio without changing its format, so that more fits in the same space. You may have not noticed it, but if the video and audio format where the same as the original, the quality had to be worse.
DVD shrink is a free program that can achieve this. It also maintains the DVD's quality.