"the burnt disc has only about 1MB used."
And is the entire video present when you play the disc?
If this is the case, it has nothing to do with Nero, the size of the file depends on Shrink only.
Maybe you left the compression settings on "manual" instead of "auto"...?...
Now if a part of your video is missing on the disc, maybe it's a burning problem.
Please tell us if the entire video is playable on the disc.