I put about 23 avi's about 350mb in size each, onto the program, and let it convert.
How many minutes of video is each file ?
I suggest not to put more than 4 hours per DVD5 and 8 hours on a DVD9 MAX.
A dvd5 is made to hold 2 hours HQ Video, and a DVD9 I guess 4 hours HQ Video.
If you didn't cross 8 hours of video, it should have fitted on a DVD9.
But without seeing the full conversion log, I have no idea what happened.
Provide us a full conversion log, and let's see what happened.
I think you are pushing ConvertXtoDVD to it's limit, trying to fit so many minutes of video on 1 media !
My 2 cents...