Dunbar's number isn't quadratic any more than the table of elements is quadratic. We can make fairly good approximations of how two elements will interact with each other just by knowing properties of the elements themselves.

The analogy fails to describe people given that major past issues can color relationships, but it doesn't seem reasonable to think that the number of such major issues will grow linearly with tribe size, this those will also be sub quadratic (I am proposing that as tribe size doubles, the number of people you love/hate due to past shared experiences less than doubles).