This works - advice I use from Larry Jordan
Try doing this compression in two steps, because the quality of the 120 minute setting is somewhat low. Compressionists call this "Pre-Compression," which means to prep the file into its final format before compressing it. When you need to create multiple compressed versions of the same file, pre-processing can save a lot of time.
After doing a QuickTime export of your sequence choosing "SELF-CONTAINED" in options:
1. Resize your HD movie to SD using Compressor but don't compress it.
* Encode it to ProRes HQ
* In the Frame Controls tab, turn Frame Controls On, set Output Fields to Progressive, Deinterlacing to Better, and turn Adaptive Details Off.
* In the Geometry tab set image size to 853 x 480 using a custom 16:9 aspect ratio
2. See how your movie looks. It WON'T look as good as HD, as the image resolution is seven times smaller. But it should look OK.
3. Then, compress the resized image to MPEG2 using the DVD Best Quality 120 minutes setting.
This two-step process takes a bit longer, but should provide a lot higher quality
Also if you are working on a multicore Mac like a 4 core iMac, go to qmaster preferences in System Preferences and turn them on and then in compressor choose the name you have for your rendering farm or whatever you call it and use that to compress your video. It speeds things enormously