Most engines use carburetors larger than they need and run the engine in the lower part of the rpm curve...If an engine will run 9000 rpm and you select a prop that will only turn 7000, the last part of the throttle opening makes little difference in the power output, the engine won't turn that fast..
If you look down the venturi into the carb, you will see that the size of the opening changes more during the first part of opening the throttle than it does after about 3/4 open, because of the angle of the disc and the size of the shaft..A smaller carb matched to the maximum rpm of the engine would be much more linear....how come nobody does it ? Who knows, maybe a great big carb looks better on the engine than a smaller one...I tested a US 41 and some other engines with carbs from 3/8 venturi to 5/8 venturi, very little difference in max rpm.....