Saul was tall and David was an average boy-when he put the armor on, it fit him perfectly it should have been a little big on him- Saul just wanted him to wear it for protection when he saw that it fits him well-he became suspicious that maybe this boy will become his successsor but he kept quiet because it seemed very strange to him- how can the armor fit him? when David saw that Saul's facial expression changed from happiness to tension he took the armor off and said to Saul- I don't need this, I'm uncomfortable- I'll go without it he took his slingshot- prayed to G-d, and hit Goliath through his armor, directly on his forehead and Goliath fell- when he fell David took Goliath's sword and cut off his head 