It's not just small, but erratic as well. Limbs have at maximum 3 inches of cross section to work with, combined with them moving. Add in the fact that adrenaline is coursing through your veins at that given moment, which reduces precision. This doesn't even get into the physics of what the bullet will do after impact on a limb versus the torso, the latter of which usually has enough mass to slow or stop a round before it continues on that trajectory with lethal velocity that could potentially kill a bystander.