Hitting someone square in the forehead with a rock, in the Bronze Age, was a quick way to kill them regardless of size. There's a reason this image

is both iconic and incredibly triggering to the IDF. You whip that thing around hard and fast enough, and you're going to crack a head.

So the whole biblical story might be based on something that actually happened

I don't find the story of a young, spry soldier with a bit of luck and some good aim thwacking a rival warlord with a rock implausible in the slightest. Its all the propaganda packed in around the story, what with David having some sort of euphoric epiphany and the rock being magicked by God to score the killing blow, that causes folks to roll their eyes in disbelief.