The book Sapiens has a few pages on this. Essentially, the thought seems to be that we're an edge case that's been able to jointly optimize for intellect/large

The book Sapiens has a few pages on this. Essentially, the thought seems to be that we're an edge case that's been able to jointly optimize for intellect/large brains and small efficient bodies by overcoming the limits on brain size imposed by vaginal birth. This in part explains our relative immaturity at birth. We're vulnerable and require constant care for the first few years of life. Compare this with other mammals such as calves which are able to walk roughly at birth.