Looking at the brotli API, I believe I spotted the source of our disagreements. Brotli separates out the window size from the quality, but in zstd the level implies the window size, unless you use advanced parameters. For example, level 3 has a 1 MB window size.

When you are comparing brotli and zstd, are you focusing on the highest quality/compression level, but varying the window size?

For us, lower compression levels (1-3) are our primary focus. We care about the highest levels for benchmarks, but it isn't our focus. When we are benchmarking zstd we focus on the lower levels, since they are the most important to us. When we think about a smaller window size, we generally think about faster compression.

When we comparing against brotli, the question we're asking is, how does brotli compare to level 3. When you're benchmarking zstd, I suspect you're asking the question, how does the highest zstd level compare to the highest brotli quality for the same window size, is that right?