Real air resistance is a force proportional to cross sectional area and speed (or proportional to the square of speed at high speeds), and heavy objects accelerate less for the same force, so that's where the intuitive idea that the heavy object should fall faster comes from. But this is much more expensive to calculate, even approximately, so for efficiency, physics engines don't do that by default.

Real air resistance is a force proportional to cross sectional area and speed (or proportional to the square of speed at high speeds), and heavy objects accelerate less for the same force, so that's where the intuitive idea that the heavy object should fall faster comes from. But this is much more expensive to calculate, even approximately, and different games will have different needs in terms of speed/realism trade-offs, or different types of fluid interactions they need to model. So for efficiency, physics engines don't make any attempt to model this messy phenomenon by default — they just give you a cheap way to make things slow down and stop jittering.

[RequireComponent(typeof(Rigidbody))]
public class AirDrag : MonoBehaviour {

    public float dragCoefficient;

    // You could calculate this automatically from the colliders / bounds,
    // or even recompute it for each direction of travel.
    // Tumbling bodies are left as an exercise for the reader. 😜
    public float crossSectionalArea;

    [SerializeField, HideInInspector]
    Rigidbody _body;

    void OnValidate() {
        if (_body == null) TryGetComponent(out _body);
    }

    void FixedUpdate() {
        Vector3 vel = _body.velocity;

        // Assume high speed flow, drag proportional to square of speed.
        Vector3 drag = -vel * vel.magnitude;

        _body.AddForce(drag * dragCoefficient * crossSectionalArea);
    }
}

[RequireComponent(typeof(Rigidbody))]
public class AirDrag : MonoBehaviour {

    public float dragCoefficient;

    // You could calculate this automatically from the colliders / bounds,
    // or even recompute it for each direction of travel.
    // Tumbling bodies are left as an exercise for the reader. 😜
    public float crossSectionalArea;

    [SerializeField, HideInInspector]
    Rigidbody _body;

    void OnValidate() {
        if (_body == null) TryGetComponent(out _body);
    }

    void FixedUpdate() {
        Vector3 vel = _body.velocity;
        float speed = vel.magnitude;

        // Assume high speed flow, drag proportional to square of speed.
        Vector3 drag = -vel * speed * dragCoefficient * crossSectionalArea;

        // How much force would be required to stop the object entirely?
        float limit = _body.mass * speed / Time.deltaTime;

        // Ensure we never accelerate more than that (no reversing direction).
        drag = Vector3.ClampMagnitude(drag, limit);

        // Apply the clamped drag.
        _body.AddForce(drag);
    }
}