I dunno. If they really were trapped in a Prisoner's Dilemma, then the equilibrium would be that they get big, I think (unless there's a "repeated games" angle

I dunno. If they really were trapped in a Prisoner's Dilemma, then the equilibrium would be that they get big, I think (unless there's a "repeated games" angle here). Yet what we actually observe, of species that get isolated on islands, is that they do get small.