Well, for one thing, converting the LD video from 29.976 fps to 23.976 fps makes a boatload of difference. My earlier attempts with just the regular video and the compressed 5.1 soundtrack filled the disc at a lower bit-rate. Now I've been able to have the PCM, DD, and the video at a higher bit-rate. The key difference is not only fewer frames to compress, but that the frames, themselves, have much "cleaner" images (i.e. no "ghosting" from movement between frames.) I also ran the files through a dot-crawl filter. The better the master, the better the compression. You know, "garbage in, garbage out."
And, as I said before, everything is mastered to as high a level as it can possibly go and still fit on the disc. My final project came out at 4.69 GBs.