Johann Hari wrote an interesting article about population growth in The Independent about a month ago, and whilst he discussed the view that humans are a "parasitic infestation", and concluded that reproductive choice is one of the main issues in controlling such phenomenal growth -
The UN Fund For Population Activities has calculated that 350 million women in the poorest countries didn't want their last child, but didn't have the means to prevent it.
He also highlighted what I feel is the most important answer to this question -
To be responsible for as many gas emissions as one British person, a Cambodian woman would need to have 262 children.
Quote from Crackdown
Is having a large family an eco-crime?
A large family does not necessarily have a devastating impact on the environment, but the choices they make do. A large family in the Western world, who live a 'typical' lifestyle, will have significantly more impact on the planet than those who have fewer children, and those who live in a more eco-friendly way.