8" vs. 9" convertor

I have a hard time believing that tq multiplication is better on a small-er diam cter.
The tq is developed by the centri force generated of the fluid leaving the impeller fins. Bigger diam, more force.
What is intrinsically linked to cter design is the rotating weight of the cter.
So how much does rotating weight play in the change from a 12" to 10" cter that improved ET? The weight & the stall cannot be separated; & the reduction in rot weight occurs where it makes the biggest difference: on the circumference.
If there is an 8" & a 9" that are 4000 rpm stall, I would use the 8".