Perhaps it's the use of a dictionary? As far as I'm aware, tar, zstd, xz do not use one by default, it's an extra set of hoops to create a training set, create

Perhaps it's the use of a dictionary? As far as I'm aware, tar, zstd, xz do not use one by default, it's an extra set of hoops to create a training set, create the dictionary, use it for compression, and pack it away somewhere so that it's available for decompression, and then actually use it for decompression. If that's all being done by 7zip just by passing -md=64m that's pretty cool. Edit: Ahh, I was confused. Neither require a separate training step. Zstd offers an option to do a training step. Both always use dictionaries with a default size that can optionally be changed.