For many illegals, 7 crammed in a 3 bedroom house with water and electricity is a step up, and they’re willing to have 2 more kids. For Americans, relying on food stamps to supplement part time or irregular work, they aren’t willing to have more kids.
If more American men were employed, I suspect their girlfriends and wives would be more willing have another child.
There's also the fact that most Americans actually devote time and resources to raising, educating and disciplining their children, all alien concepts in the barrio.