Chances are that you avi file IS only 650MB, but when nero converts it to SVCD then the size is much larger because SVCD uses a fixed video resolution and bitrate per second, so nero needs to convert it first. This will result in teh movie being much larger than nero can put on a single cd.
SVCD can only fit about 50/54 minutes of video on a single cd.
VCD can fit 74/80 mins of video on a single cd
Try using something like TMPGENC to encode the file as an MPEG2 file first and then use VCDEASY to convert the mpegs into disc images and burn them.
Using TMPGENC, you will need to cut the resulting MPEG into sections approximately 700/760MB (for 74min and 80min cd respectively) in size before using VCDeasy to put them on cd
These tools are both free, and relatively easy to use.