It was pointed out elsewhere that I'd encoded it too big anyway, and dragging the two files into ImgBurn confirmed that even the main video and audio on their own were too big for a disc, suddenly coming out at 8.2GB. I still had the bitrate calculator open, and saw that I'd accidentally selected DVD-10 instead of DVD-9.
So I re-encoded with the correct bitrate, and ended up with a 7.1GB video file, and an 8.27 GB timeline. So the timeline is still a gig bigger than the assets are. Although it does fit on a disc.
It's not measuring the assets by whatever method makes a DVD 7.96GB and the timelines by whatever method makes it 8.5GB, is it?