Just to clarify a little what others have said, a dual layer disk has a semi transparent recording layer, and another recording layer under it. There are two recording layers, but they are read and or written from one side of the disk.
The reason that it would only allow 6 episodes is that you were trying to convert it to dvd format, which converts the avi to mpeg 2. Standard dvd video format can only use mpeg formats. You can burn it as a data disk as suggested, and it will burn the files as avi files, but dvd players will not play it (unless it is a multi format dvd player that plays that particular kind of avi file). It will however be playable on your computer and you can read the files off it, no problem.
If you want it to be playable on normal dvd players, you will have to convert to dvd format, and it is normal for the file size to grow substantially like that. mpeg 2 is an older format that does not have the high quality compression that newer/better avi formats have, but that is what you have to use with standard dvd video format for normal standalone players to use it. As mentioned, you can reduce the bitrate to get more video, but if you try to fit that much video on a single disk, it may be at the cost of a severe loss in video quality. Normally you can just keep adding video files, and nero will ask you if you want to reduce quality (bitrate) to allow it to fit. If you reduce it enough times, I think the final time or two it will cut the resolution in half too.