Because DVD-Video requires an inefficient, older encoding scheme and iDVD won't let you use low bitrates. 7 hours on a DVD is a little over a megabit, which is probably ok for animation; other apps (Toast, VisualHub) will let you do this.
If your set-top DVD player can play the files you downloaded (probably some MPEG4 variant), then you can burn a data DVD through the Finder.