Don't know if it makes a difference, but after checking my history, I was using -crf 21 and bumped it down to -crf 18. My brain is telling me I read somewhere to change cfc in steps of 3, but I'm not sure how reliable that is. So I never tried 20.
I couldn't stop being curious, so I just ripped the blu-ray for "Big Fish", which was 28.9GB raw. After running `ffmpeg -i big_fish.mkv -c:v copy -c:a flac new_big_fish.mkv`, the resulting video was 25.5GB...indicating to me that FLAC compression is worth it, since there's no loss in quality.
FLAC is also just really well-supported pretty much everywhere nowadays, so it's kind of worth it to me regardless.
Your test is possibly invalid; without any -map options, ffmpeg will not copy all of the source streams. I don't have big fish, but Les Miserables(2012) I found that flac was about twice the size of the DTS source, which met my expectations:
