Why is the DVD capacity for iDVD shown as 4.2Gb when my DVDs show a capacity of 4.7Gb?

Question

I've recently discovered iDVD 7.1.2 and when trying to write out a file, the option for a single layer DVD is 4.2Gb, although my DVD's show a capacity of 4.7Gb.
Why the discrepancy please?
(sorry if the question's been asked before, the ASCs are new to me)

Answer 1

iDVD doesn't care about size (no joke here).  It's only concerned with playing time.  iDVD will encode and compress the files to fit the DVD disk depending on the encoding method chosen. The following was taken from this Apple  document: iDVD '09 (7.x): About iDVD encoding settings
The encoding settings in iDVD are the following
◼ Best Performance (for projects less than 60 minutes in length).
When you select Best Performance, iDVD encodes your video in the background as you’re working on your project instead of waiting until you burn it. This can significantly reduce the amount of time it takes to create a finished DVD. This is a good choice if your project is an hour or less in duration (for a single-layer disc).
Best Performance encodes your video at a preset bit rate that produces great quality for projects that can easily fit on your disc. If you have a larger project that needs to be squeezed onto a disc, you should select one of the other encoding options.
◼ High Quality 
This is the best encoding option for larger projects that are between 1 and 2 hours long (for a single-layer disc). With this setting, iDVD chooses the best bit rate possible for the amount of data you need to fit onto your disc. It’s the only option that “squeezes” all the data onto a disc at a bit rate that ensures a high video quality in your burned DVD. Because iDVD does not encode in the background with this option, it takes longer to burn your disc. The encoding process begins when you burn the DVD instead of when you start working on your project.
◼ Professional Quality 
The Professional Quality option uses advanced technology to encode your video, resulting in the best quality of video possible on your burned DVD. You can select this option regardless of your project’s duration (up to 2 hours of video for a single-layer disc and 4 hours for a double-layer disc). Because Professional Quality encoding is time-consuming (requiring about twice as much time to encode a project as the High Quality option, for example) choose it only if you are not concerned about time.
So depending on the playing time of your project select the encoding method that will best suit it.
Note:  if there is a lot of fast motion in a video the Professional Quality will determine which portions of the video need the least amount of compression and which can stand the most in order to produce the best overall qualtiy possible.
OT