It is both nurture & nature. Familial & outer environments influence what a child will become. Besides familial & outer environments, educational & socioeconomic factors influences what children will become. Children in small family environments tend to have parents who are more involved in their lives. In the small family environment, parents play a substantial role educating their children through teaching, discussion, & other forms of interaction. Small families tend to be more socioeconomically affluent & educated which further substantiate the importance of nurture for children.
In large families, it is more nature in terms of children's development. In large families, children are left to their own devices. Child neglect is commonplace in the large family environment. Parents of large families don't have the resources to raise & care for their children so children must learn to navigate their environment early. Children who can't navigate their familial environment early, fall through the familial cracks as their parents aren't there to care for them. Parents of large families don't interact, discuss, nor teach their children- they leave these to oldest/older siblings. Parents of large families just have the children & give them to siblings to raise. Also, large families tend to be socioeconomically poorer, even impoverished & parents are less educated.
In large families, it is the nature premise. Children learn their innate psychology & act upon it. The stronger children survive while the weaker ones become overwhelmed & psychologically, even psychically destroyed in one way or another. In small families, there is more nurture, parents are there for their children & look out for them.
Beyond family size, educated/affluent parents of the solidly middle, upper middle, & upper classes placed emphasis on nurture as they know that children need solid parental examples in order for them to become well-rounded, functioning adults. Less educated/poorer parents of the lower middle, working, & lower classes don't consider nurture important, they believe in nature i.e. children teaching themselves how to navigate their particular environment.