[2:251] And they routed them, by the leave of God, and David slew Goliath; and God gave him the kingship and Wisdom, and He taught him such as He willed.
God frightened the enemies with Saul since He has given him such a large body, but at the time of the fighting He made the victory at the hands of David. As the story goes, he was of middling stature, not large in body, a compact person. He had no weapon with him but a slingshot, but the victory was his because the help of God (s) was with him.
And they routed them, by the leave of God so that nothing, not even a trace remained of them. David killed Goliath. Because of Goliath's bulk and size, David's victory over him was unimaginable, but as the one who spoke for them said:
He met me
with his sword drawn
and said to me,
'One of us is gone'.
[2:251 cont'd] Had God not repelled people some by means of others, the earth would have surely been corrupted; but God is bounteous to all worlds.
If mankind were to make common cause and agree with one another altogether, the weak would be destroyed by the victory of the strong. But He makes some engage with others so that by their engagement He repels a people's evil.