haha Simon
its pretty easy to understand if you actually think about it, the higher boost you run the more exhaust gasses from the motor go to the rear of the turbo meaning that less needs to flow through the WASTEgate for it to hold the desired boost. The opposite applies for low boost, less amount of exhaust gasses need to go to the rear of the turbo meaning the WASTEgate must flow more air to hold the desired boost.
In reality most rotaries run "low boost" compared to piston motors.
For my "low boost" 10psi i could of got away with a 48-50mm gate but got a great deal on a 60mm one, figured a bit of overkill can never hurt