The ULT was designed to make the trigger lighter and remove reactivity at the same time.
The smaller head diameter reduces the amount of force that the tank pressure can exert on the sear when the front chamber is empty. If the top of the pin was as large as the retro on-off pin, then reactivity would be increased using a smaller diameter pin in the lower section.
When you pull the trigger of a retro valve, you are only pulling against the force exerted on the small diameter of the pin. The top and bottom of the larger head of the pin are at the same pressure so they almost cancel each other out and the only difference in the two surfaces is the small diameter of the pin. It is the only sealing surface when the chamber is pressurized and the direction is down onto the back of the sear. This is what gives the ULT such a light trigger pull.
The amount of reactivity is dependent on force differential. The mass of the sear and trigger require a fixed amount of force to start its movement. In order for the pin to work reliably at standard tank pressures, the diameter of the pin top needed to be a certain size to provide that force.
Shims couldn't stop reactivity on a ULT with a larger retro sized top head and smaller lower diameter without making the trigger pull abnormally long.
Except for the Automag in front, its usually the man behind the equipment that counts.