I believe that’s correct – if nothing has changed from your last generation, then the new generation will be identical. But if something has changed, it will do a bunch of duplicating and remapping symlinks in the Nix store to ensure that everything plays nicely together and that you can rollback to a previous generation if needed.
So if you do a rebuild switch regularly, you will end up with gigs worth of old “copies” of things that aren’t being referenced in your current generation.
That’s what nix-collect-garbage handles – once you know your current generation is working well, you collect the garbage and recover that space, at the expense of not being able to roll back.
That’s why I think building a core system with NixOS and then having user software come from Flatpak is a nice combo for simple workstation that won’t update and bork itself, leaving my grandpa without a laptop until I can come take a look.
Edit: To clarify, nixos-rebuild-switch won’t update your Flatpaks at all – just the Flatpak service