If you convert the movie with "meduim quality" it should be a lot smaller. If you burn it with "high quality", ConvertX will always make a file that fills the whole disk. There is no option for choosing to make a high quality disk that only fills (say) up to 4 gig.
Your other options is to after converting the movie to disk, use Nero to burn it out rather than ConvertX. This gives you the option of using Nero's "short lead in" and even "overburn" features to get a few more megs out of a disc, although you need to read up on the caveats involved with these two features in the Nero forums here at CDFreaks.