Interesting--thanks for checking! I had good experiences with bzip3 compressing Wikipedia XML dumps, to the point it even outperformed xz, so I thought something similar might happen here. Compression does remain a bit of a black art, where it's hard to predict what works without trying it out.

Overall I'm still slightly biased towards using zstd as a default, in that I believe:

  1. zstd will almost always be among fastest formats for decompression, which is obviously nice-to-have everything else being equal.
  2. zstd can achieve a very high compression ratio, depending on tuning; rarely will zstd significantly underperform the next best option.

Overall this is a pretty good case for using zstd by default, even if in some cases it's not noticably better than other formats. In your case, xz seems to be just as good.