Final answer:
The legalization of slavery in Georgia in 1750 and the economic incentive of land grants tied to slave ownership were pivotal in the growth of the slave population, further influenced by the cotton and sugar economies and the domestic slave trade.
Step-by-step explanation:
The most significant factor that affected the growth of the slave population in Georgia was the legalization of slavery in 1750 by legal decree.
This decision led to an increase in large plantations and influenced immigration patterns, as land grants were tied to slave ownership, incentivizing the holding of more slaves for more land.
The internal demand for enslaved labor grew with the expansion of the cotton and sugar economies, alongside the domestic slave trade after the international trade was banned in 1808, leading to a significant increase in the slave population in Georgia.