There are many examples given in this chapter of the impact that humans had on large animals. In general, Diamond is arguing that people caused the extinction of large animals when they reached the Americas as well as when they reached Australia/New Guinea.
As an example, on p. 42 of the paperback edition of the book, Diamond points out that Australia/New Guinea had "its own suite of diverse big mammals." It also had a 400 pound flightless bird that resembled an ostrich and marsupials the size of cows.
As another example, on p. 46, Diamond gives examples of megafauna that lived in the Americas before humans came. Among them were such things as elephants, horses, camels, and lions. All of these became extinct soon after humans reached the Americas.
Diamond argues that humans hunted these animals to extinction and/or hunted the prey that carnivores relied on, thus driving even less edible animals to extinction.
It was no coincidence that big animals became extict when humans arrived, They were either killed for food, or to keep themselves safe from these unknown animals