In other words there is so much illegal immigration, it is increasing the population of the states where they go. In California, about 25% of the population is foreign-born.
Look back at the article and you will notice the growth in Arkansas. Overall, those numbers don't look to large, but in actuality, most of the illegal growth is along the state's western edge where they are processing chickens and building the boom in the NW corridor. So that growth is confined to a narrow area.