I suspect that interframe compression is not used in AVIWriter (but I may be wrong). You may try to use VideoFileWriter from Aforge.Video.FFMPEG instead:
var writer = new VideoFileWriter();
writer.Open("test.mpg", 320, 240, 30, VideoCodec.Default, 1000);
// add your frame
writer.WriteVideoFrame(frame);
Remember to put dlls from Externals/ffmpeg/bin from AForge zip into your output directory.