That's how large they are. I have chosen to store them at full resolution, as I want to be able to have the best possible PQ for projection, when going from DVDs. (Imagine a Blu-ray disk at 30GB ...)
Using MactheRipper you can strip trailers, behind-the-scenes, interviews, irrelevant audio-tracks, etc, in order to slim the file. But I haven't bothered.
Check your DVDpedia program, look under EDIT in the menu bar. You should see "Add Multiple DVDs."