[klipt]

That's a prisoner's dilemma situation. As a group, smaller islanders are less likely to run out of food (the cooperation strategy). But as an individual, a bigger islander can beat up the smaller ones and take their food (the defection strategy).

Of course if everyone defects, the group as a whole is worse off.

[FooBarBizBazz]

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.