David was originally to be placed on the top of the Duomo, and Michelangelo planned for that. The perspective of the body is slightly skewed--the head enlarged with the shoulders, so they would look in proportion while viewed from far below.
But the left hand is enlarged also, perhaps an effort to show the tense grip and the veins pulsing.
This isn't a shaky hand. After all, he's well practiced with the sling. His brow is focused on his target and his hand shows every intent in swinging into hand-eye timing and coordination. Within a second after the moment captured in the statue David loads his sling, shifts his body weight toward his rear, then counter-shifts again as his he arm swings and releases...
He knows his aim will be right-on.
The Goliath will be dead.
Only after he accomplishes this, will his hand relax once again.
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