Have you considered rendering your project as a .m2v and .wav file instead of QuickTime? Doing so will allow you to choose the quality of the rendered DVD video file rather than letting DVD Studio Pro re-encode your QuickTime video to whatever settings it has as its default.
You may already know this, but when you import anything other than a .m2v (or mpeg2) codec into DVD Studio Pro, the program has to re-encode the video and audio to formats that work on a standard DVD. This means that you're QuickTime file is being re-encoded by DVD Studio Pro into a .m2v video and .wav audio file (DVD's require that the audio and video streams be encoded separately).
I personally don't give many props to DVD Studio Pro's ability to re-encode a video to an MPEG-2 file for playback. After all, the program was meant to allow editors to make menus and links not to encode video. Instead, I use either Adobe's Media Encoder or Apple's Compressor to make my final export into an MPEG-2-DVD file (or in other words a .m2v and .wav file) and then import those files directly into either DVD program. There are presets in both compression programs that will allow you to render your video into a format ready for DVD. I would also recommend that you turn on all of the highest quality settings in your compression program (in Media Encoder this includes checking the maximum quality render box and the frame blending box) and that you turn your bitrate up to 9 (the maximum for any DVD). The bitrate setting is going to make the real difference in the way your final DVD looks. If your video is longer than half an hour, you may have to turn your bitrate down to 8 or lower in order for it to fit on a standard DVD. However, the higher the better so it's worth the time to test your bitrate and file size to get it just right.
Hopefully this helps!