simple answer

I also think the case is interesting, but has a simple answer. The cases have nearly the same moral value. "Nearly" because by making a promise someone is making a commitment and the promisee could learn something about the promiser if they had failed to keep their promise. If they had broken their promise, then they would have made the matter worse than if they had not made a promise at all and not gotten the coconut. In this situation, the added moral (dis)value comes from what could have been done, but wasn't. Moral value crosses worlds
Secondly, it isn't clear that the tall fellow wasn't under an obligation in virtue of being able to feed the short guy. In both cases their is still the obligation, that one is special in virtue of the promise doesn't seem to make a big difference to me.