There are probably some things you can do to improve that a little like extended play settings etc. You might also want to try making it too large and write the image to hard drive, then use recode to compress it (haven't tried that yet but I am going to sooner or later). In short though, even doing things like that you probably won't get more that two on a disk (depends on how big the files are). Thats just the way it works. The files have to be converted to dvd compliant files which are going to take a certain amount of space for so much video, period. Thats why nero has nero digital (mpeg4) so you can get more stuff on a disk. Nero is trying to get companies to make dvd players nero digital compatible (I think thier may be one) but untill they start making dvd players that can play xvid and stuff like that you are only going to get so much video on a dvd that will play on a standard dvd player.
Its kind of like mp3 and standard cd format. You can get 100 songs on a cd in mp3, but even if the source is mp3, if you convert them to normal cd format, you are only going to get 15 (give or take) songs on a disk period.