Why Is 480p VBR MPEG2 Limited to 8Mb/s?
 

I have tried a multitude of different settings in AME CS5 to export a 720x480 29.97p .901 PAR, 2 pass VBR and the 30s video never exceeds 30MB in size. I even rendered an uncompressed AVI and imported into TMPGEnc XPress 4 and this program is doing the same thing with MPEG2 export.

AME says the file should be 72MB at a 20Mb/s target bitrate.

The only way I can get a higher bitrate file is by using CBR or by exporting as TS (transport stream).

I think you may have hit the maximum limits of standard-definition MPEG-2. Remember, MPEG-2 is highly lossy to begin with. Plus, standard-definition DVD is limited to a maximum total bit rate (that includes not only video, but also audio and various other tracks) of 10.08 Mbps. The 8 Mbps maximum for video is to keep the total bit rate within the DVD-Video specs.

On the other hand, 29.97p is not really supported by DVD-Video. Instead, 29.97 fps video on DVD must be encoded as interlaced (or 29.97/59.94i).

And 20 Mbps SD video is already nearly as high of a bit rate as DV-AVI (which has a CBR bit rate of 25 Mbps).