Turbocharger turbine A/R Radius value

[UNCCgearhead]

In terms of turbocharger turbine section, A/R is the ratio of Area at the begining of the volute to the Radius which is the radial distance from center shaft to the center of the aformentioned area. For a given mass flow rate, If A/R is large, shaft speed is lower but backpressure is less. If A/R is small, then the opposite is true. Here is my question.
Why does increasing R increase shaft speed with the wheel size and all other things constant?

Thanks

 

[Prodlgy]

increasing R doesn't increase shaft speed it increases the torque the shaft is able to develop