Brotli defines memory use as log2 on command line, i.e., 32 MB = 1 << 25

zstd uses a lookup table where the number given by the user is mapped to a decoding-time memory use. The user just needs to look it up if they want to control decoder memory use.

If one benchmarks zstd with `20` and brotli with `20`, zstd may be using 32 MB of decoding memory, where one is specifying 1 MB for brotli. By default zstd tends to use 8 MB for decoding (however it is variable with encoding effort setting) and brotli 4 MB for decoding (not changing with the encoding effort setting).