Not sure about snap, but at least for Flatpak, it has runtimes and runtime extensions that Flatpaks can share, which already reduces the individual Flatpak size. But further more, all runtime, extensions and Flatpaks live in a single OSTree repository on the system, which does automatic de-duplication on the file level.

While probably still less efficient than a properly maintained and integrated distro, where all software uses a single set of shared core libraries, its actually (in the Flatpak case) quite optimized & has additional benefits, such as installing a set of apps that don't share a common set of required library versions, which would normally be impossible.

> These formats might be OK for desktops, but are absolutely awful for servers, containers, etc., which, incidentally, are where Linux dominates.

Containers have already won the war on the Linux server, there's no point Flatpak etc trying to compete here. Additionally, desktop software is a complete design paradigm to server software. I wouldn't even say they make sense to be in the same package manager.

Although they do share tech, ostree, namespacing etc.