Do you care about the size that much? I guess QT X used the H264 codec to transcode the video to and AAC for the audio? You can find out via opening the transcoded file (8GB) in QT X and open the Movie Inspector (Command + I) and see for yourself and maybe post the results here. As that file is only temporary I guess, can you live with the larger size of the output, as you only use it for iDVD and trash it afterwards?