Nero Vision is re-encoding your divx into mpeg2 since you are making a dvd. You can burn your xvid or divx straight onto a dvd as data files if you like. If your player is capable of playing divx, it should be able to run these divx encoded files from your disk. Many newer players have this capability.
Divx and Xvid are both more highly compressed than mpeg2---that is why your 130mb files turn into 1.23gb. They may also be a different resolution.
The other solution is to encode all of them to dvd compliant mpeg2 format, then run the whole thing through DVDShrink to get it to fit onto a single dvdr. You would be processing the video twice, so I don't know if the quality would be acceptable for you. If you go this route, I would start with the xvid files straight to dvd compliant mpeg2. And I would use a better encoder than Nero if at all possible.