How big will my long coat chihuahua get?

My puppy is a 17 week old, long coat deer head chi, and weighs 4 pound 11 ounces. The vet guessed 10-12 pounds. But he just does not look like he is or will be that big. I was shocked at his weight. I have no background for him. He was a rescue. And is supposedly pure chi, although I have never seen the parents. I have a 9 pound chi/min pin mix and she makes him look so small. How much bigger will he get? When will he stop growing? He still needs to fill out, so will that be how he grows mainly from his age?