According to Guns, Germs, and Steel, why do hunter-gatherers control their population size?
You can find the answer to this by reading in Chapter 4. On p. 89 in the paperback edition of the book, Diamond talks about why (and how) women in hunter-gatherer societies need to control their population size.
Diamond says that hunter-gatherers' nomadic lifestyle requires them to have their children's births spaced out more widely than people in sedentary societies must. In a sedentary society, it is not a real problem if a woman gives birth to a child while her older child is two years-old. In a nomadic society, it is a problem. She would have to carry the infant, but the two year-old would not be old enough to walk long distances fast enough to keep up with the group.
Because of this, hunter-gatherer societies have to space births out more widely. This controls and limits the size of their populations.