It's not really the length of the shanks, per se, that determines poll pressure.
Look at what has to happen for poll pressure to be applied. You pull back on the reins, with causes the shanks to rotate and move back. As the shanks move back, obviously, the purchase moves forward. When the curb chain tightens as far as it can, this movement stops, and you get the leverage effect in the mouth, pulling the mouthpiece down onto the tongue and bars. It's the forward movement of the purchase, however, which pulls downward on the bit hanger, this tightening the crownpiece and applying poll pressure. But how much does the purchase actually move? Well, not much....
No one sane uses a bit like the above photo shows. If you crank back that far and that hard, you can see how the headstall tightens behind the ears. But if you pull back with less force--or if you have the curb chain adjusted tighter--that can't happen. You'll get some poll pressure, but only a negligible amount. Look at the first photo, again, and see how their is curb rein contact (bit rotation), but still looseness (slack) in the bridle.
If you really want substantial poll pressure from a bit, use a gag/elevator/draw, which is designed specifically to have this effect, by shortening the bridle as the reins are pulled.
Edit: Great visual!