The Non-Miraculous Explanation:
Malcolm Gladwell, author of David And Goliath: Underdogs, Misfits, And The Art Of Battling Giants, points out that clues in the verses where Goliath is introduced suggest that he suffered from acromegaly -- the same pituitary disorder that's plagued extraordinarily large individuals throughout history. First off, there's the fact that Goliath lumbers his way onto the battlefield led by an attendant. Why would the Philistines' mightiest warrior need an attendant? Possibly because, as is often the case with an enlarged pituitary gland, Goliath couldn't see for shit. This is further reinforced by how, once he finally spots David, Goliath taunts him with, "Am I a dog, that you come at me with sticks?" David was only carrying a single shepherd staff, which Goliath refers to in the plural -- double vision is also associated with the disorder.
Then he inexplicably called David "four eyes," and David really got pissed.
And then there's David. He was a shepherd with years of practice defending his flock from lions and wolves with his sling. And a sling was not some primitive weapon at the time -- slingers were the artillery units of a Biblical army. Add in the fact that the stones David picked up in the Valley of Elah were "twice the density of normal stones" due to the chemical makeup of rocks in the area, and he was packing a weapon "roughly equal to the stopping power of a [.45 caliber] handgun." Goliath, an already handicapped opponent, brought a sword and a spear to a gunfight.