In the dedication section in the beginning of Larry Niven's "The Ringworld Engineers", Niven writes:
Freeman Dyson (Freeman Dyson!) has no trouble believing in the Ringworld (!), but can't see why the engineers wouldn't have built a lot of little ones instead. Wouldn't it be safer. I hope the answer I've given in this book is satisfactory.
I read the book but I don't understand what is the answer. What is Niven talking about? What answer does the book give to this question?