NFL players aren't bodybuilders. They don't put on weight just for the sake of putting on weight because it will slow them down.
Smith is 1 inch taller, he could probably put on 10 pounds, but he's not going to put on 20 and still be the same type of player.
Jack's legs and midsection are both considerably thicker, which as I am sure you are aware is where the majority of muscle weight is carried in athletes.