Full Idea
Rawls objects to utilitarianism because it fails to take seriously the separateness of persons (because there is no overall person to enjoy the overall happiness). But Nozick thinks Rawls does not take the separateness of persons seriously enough.
Gist of Idea
Utilitarians lump persons together; Rawls somewhat separates them; Nozick wholly separates them
Source
comment on John Rawls (A Theory of Justice [1972]) by Adam Swift - Political Philosophy (3rd ed) 1 'Nozick'
Book Reference
Swift,Adam: 'Political Philosophy (3rd edn)' [Polity 2014], p.32
A Reaction
In this sense, Nozick seems to fit our picture of a liberal more closely than Rawls does. I think they both exaggerate the separateness of persons, based on a false concept of human nature.