Final answer:
The increase in brain growth during the first few years of life is primarily due to the increase in size of existing neurons and the proliferation of neuroglia, which leads to the formation of new folds in the cerebral cortex.
Step-by-step explanation:
The increase in size of the existing neurons and the proliferation of neuroglia contribute to the growth of the brain during the first few years of life. These changes take place within the cranial vault and result in the formation of new folds in the cerebral cortex, increasing the brain's surface area.