I'm so sick of this claim. Nix allows you to keep old versions of things installed, but you certainly don't have to.
When I switched from Debian to NixOS a few years ago, I installed it on a separate subvolume, and it ended up taking almost exactly as much space as Debian did (about 12 GiB with gnome and everything else). And really, what would you expect? It's nearly all the same code, just organized differently in the filesystem.
P.S., you can check the store usage of the current system profile with `nix path-info -Sh /run/current-system`.
When I switched from Debian to NixOS a few years ago, I installed it on a separate subvolume, and it ended up taking almost exactly as much space as Debian did (about 12 GiB with gnome and everything else). And really, what would you expect? It's nearly all the same code, just organized differently in the filesystem.
P.S., you can check the store usage of the current system profile with `nix path-info -Sh /run/current-system`.