Immigration and emigration is typically done with the purpose of looking forward to a better and more prosperous life, a more comfortable life where the money you earn can last you a pretty long time and buy you enough thing to live more comfortable than where you used to live. Therefore, when you move to a prosperous country, having children is not something you’ll worry about given to the commodities you’ll have and the easy access to baby food and baby clothes and stuff, therefore, when people emigrate/immigrate to more prosperous countries, they’re comfortable with the idea of having more children, and that leading to population growth.
