It seems like the "lightweight" they seem to be concerned with is RAM, not disc. I can't really comment on disc space usage on my own NixOS system, though my "thin client" skin does fit nicely on an old Chromebook, I think that guy has 32GB of storage.
OTOH I have an ex Chromebook with 16G of storage that I meant to run NixOS until I discovered the initial install needed almost half the disk so I figured it was doomed the moment I tried to update while also having some space used in home. It runs Alpine now, very comfortably.
I’ll just say as nix Darwin user the default vanilla install does not garbage collect well and every 2 months or so I have to hack around nuking shit in my system to free up space. Maybe NixOS itself is better managed and it’s an idiosyncrasy of nix-Darwin. Maybe not