One aspect of Humanism in the Italian Renaissance was an interest in Classical antiquity (ancient Greece and Rome). Ancient Greek and Roman sculptors made many large nude sculptures which demonstrated a knowledge of human anatomy. This focus on the human figure fits with Humanism's emphasis on human potential for great achievements.
Both versions of David show a mastery of human anatomy and use the Classical contrapposto pose, with the figure's weight on one leg forming a gentle S-curve. However, the nudity of Donatello's David references Classical nude sculptures (like Doryphoros by Polykleitos), while Verrocchio's David is clothed.