This expression indicates which one of the two groups a particular person/thing belongs to. The two groups are compared in terms of the degree of their attributes. The speaker gives a certain description about the person/thing, but s/he does not think the degree is extreme.
I'm relatively fat among my brothers [and sisters].
Ali is one of the taller students in the class.
I think Mana's Japanese is above average.
Look toward her.
Tokyo has a larger population.