David did not remove Saul’s tunic and armour because he was too small and it did not fit him. He removed it because he was not used to Saul’s equipment and knew that it would hamper his movement and thus his ability to fight. David was not planning to fight with a sword in hand to hand combat, but from a distance with his sling.
39 and David strapped his sword over his armor. And he tried in vain to go, for he had not tested them. Then David said to Saul, “I cannot go with these, for I have not tested them.” So David put them off.