The main reason why David is so small is because is he scared. Michelangelo intended to represent David right before or during his fight with Goliath. David was probably somewhere between 12 and 15 years old. Imagine how your body would have reacted at that age if you would have had to fight the strongest soldier of an enemy nation, 1 on 1 in a battle to the death. I highly doubt you would be erect or have a semi.