Unfortunately the result of a Nix Docker image is an image that is 100+ MB for no particular reason :(
[1] https://nix.dev/tutorials/nixos/building-and-running-docker-...
What you linked is the equivalent of:
FROM scratch
COPY hello-world /hello-world
Of course that's small (hello-world is statically linked). Try to add coreutils (or any other small package) and you'll see what I mean. In my experience the size of a Docker image built with some nix packages is greater than the Debian counterpart. I don't know why though.
However I'll give you that it could be smaller in more complex examples. For example glibcLocales is for all locales which is quite chunky but your application only needs one locale.