The amount of force created when something hits the ground at terminal velocity depends on its mass, which varies with the cube of its size (with additional effects because it's not a cube or sphere). The amount of cross-sectional area over which this force is exerted varies with the square of its size. So when a small object hits the ground, that's a lot less force per cross sectional area than when a large object hits the ground. You'd expect the iPhone to take proportionately less damage than a person even not considering that it also has a smaller terminal velocity.