iDVD is complex, this site helps

iDVD FAQ

Make sure you are using the latest QuickTime, and as always, work with high quality source material (properly sized photos [640 X 480] for slides and/or properly processed digital video [ 720 X 480] for movies). iDVD 4 & 5 now use the same encoding technology used in Compressor 1.2 and DVD Studio Pro 2.0.5…the same codec as Apple’s professional video utility, ‘Compressor’.

Compressor uses a 2-pass VBR encoding method that first analyzes the entire file and then uses that data during a second pass encoding stage to intelligently distribute bit allocation per frame. The result? An optimized MPEG-2 file with consistent visual quality.

Ok, this is complicated. Good site.