you could try compressiong them in the 7z/7-zip format, it may make a big difference. Don't try compressing compressed files, contrary to popular belief if you zip a file enough times, you won't eventually get it to 1 byte
Nero also supports overburning, some people claim that they can get up to 50mb extra, but that would depend on the brand of DVD you are burning to and possibly the drive you have too.
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