Basically, if you go to that article, Bentham's utilitarianism is narrow, because it assumes simple pleasure and pain to be the two absolutes of ethics, whereas JS Mill's utilitarianism isn't as narrow.

What I personally meant by a narrow utilitarianism is a utilitarianism which uses a methodological individualism to form a societal calculus of happiness (unqualified, in the sense of Bentham), of which the maximation is taken to be the end-all of decision-making (individual as well as collective).