Justice in the city and the individual

In the passage from Book II, Socrates and crew decide to continue the investigation by the method of hypothesis, assuming that Justice is one and the same in the just city and the just individual. Why might one think that Justice is the same thing in cities and individuals? Why might one think the contrary, i.e. that justice might be different in each? If you think the former, do you think it makes sense that Justice would be easier to find in the larger? If you think the latter, is there any meaningful connection between Justice in the city and the individual, and how might one investigate that?