Trial and error here honestly is what I've found works best.
When I record with Debut, I do so at the more raw format possible, Microsoft Video 1 and PCM audio. Its a huge file but looks wonderful. Then I use Prisim to downvert to the MPEG4 and MP3 native settings.
I do this because some of the things I want to record get a better frame rate with the MV1 and PCM encoders. Its an extra step, but it works well for me. Then I can delete the huge avi (10 minutes roughly equals 1 gig), and keep the converted avi which comes it at around 20% of that size.
Hope that helps. If not, let me know. I've got some videos on YouTube that might be of assistance.