In exercise 2.4, part d), it´s important to notice that when the proposer is more generous, he generates a possitive externality: by contributing more, the proposer has to interact with more people. Also, by receiving a larger amount of money, the responder(s) now will have more money to make transactions with. Therefore, more generosity aids to have an integrated market (and that in general should make it more efficient as well).
A real life example could be investment on education. When people acquire knowledge, they become better prepared to make decisions in general. That lowers their risk (you could say knowledge is power here), which make them more propense to invest and to participate in the market.