Output Video Quality

A standard-compliant DVD supports a combined data rate of 9800 Kbits/second (Kb/s). This includes Video, Audio Tracks (if any), Subtitles (if any) and less than 1% of overhead transmission and control data. At this rate, a standard DVD-5 (4.7GB) can hold at most (about) 135 minutes of video (with accompanied audio and subtitles) but no more. Since most discs also have menus, bonus material and extra features, as well as more than one audio track, the main attraction (the main movie) is usually encoded at a lower rate. So in practice most discs have video encoded at 5000-6000Kb/s.

DVD's utilize the Mpeg-2 video compression standard to compress the video. Mpeg-2 is twice as efficient as Mpeg-1. That is, Mpeg-2 can produce almost identical quality video to that of an Mpeg-1 at 50% the filesize. (Notice that the inverse might not be true: Mpeg-2 at the same bitrate of Mpeg-1 might not produce a video at twice the quality!) Similarly, Mpeg-4 is twice as efficient as Mpeg-2. Of course this comparison assumes the same input video.

Example: A video encoded using Mpeg-1, Mpeg-2 and Mpeg-4 SP, Mpeg-4 AVC can have almost identical quality, but the Mpeg-2 file will be 50% the size of the Mpeg-1 file and the Mpeg-4 SP will be 25% the size of the Mpeg-1 file and 50% the size of the Mpeg-2 file and Mpeg-4 AVC will be about 13%.
 
While the DVD and SVCD standards utilizes the Mpeg-2 format, VCD utilize Mpeg-1. iPod supports both Mpeg-4 Simple Profile (SP) and Advanced Video Coding (AVC). Mpeg-4 AVC when used at its full power is expected to be twice as efficient as Mpeg-4 SP. However, iPod uses only the most basic features of AVC and thus the quality or efficiency of AVC output is not as impressive as it could be.
 

DVD supports video resolutions of up to 720x480 pixels @ 29.97 frames/second for NTSC and 720x576 pixels @ 25 frames/second for PAL.

Notice that since increasing the source video resolution increases that amount of information in the input, the video quality will suffer more as a result, unless you higher the encoding bit-rate (if possible.)