Jonathan, my assumption is that it's not the power of the motor but the weight of the car and the efficiency of the brakes that determines the unbraked towing limit - in other words how fast you can stop! My Berlingo is mainly a box on wheels so relatively light in weight when compared to it's size, hence the low limit for an unbraked trailer.
Incidentally when I consulted the owners manual the braked towing limits were specified against a percent figure which turned out to refer to the steepness of the road, although what happens if I meet a steeper hill I don't know. I mention it because even the Citroen dealer didn't seem to know what the figure meant! One person suggested it referred to the amount of braking applied to the trailer (apparently not true).
Peter