Why did David take off Saul's armor before facing Goliath?

Answer: He was not used to it

David tried to walk in Saul's tunic, helmet, and armor but said, "I cannot walk in these, for I am not accustomed to them."

“Then Saul clothed David in his own tunic, put a bronze helmet on his head, and dressed him in armor. David strapped his sword over the tunic and tried to walk, but he was not accustomed to them. “I cannot walk in these,” David said to Saul. “I am not accustomed to them.” So David took them off.”— 1 Samuel 17:38-39 (BSB)