Most european and westernized countries have low ratios. Whereas 'developing' countries seem to have a lot of children born to each mother in comparison. Some correlate poverty with family size, but at least in western countries where the number of children you have is largely a personal (our a couple's) choice this does not hold. In largely agricultural societies it may be true that the more children you have the more work you can do so the more likely your family is to survive, despite having a few more mouths to feed.
* rates estimated for 2005 according to source