it's really not because of better flow per mm of bore
it's because many CV carb'd bikes come equipped with way too much CV carb
and a mechanical slide carb does not forgive simply because it is possible to over carb with a mechanical slide
and it simply is not possible to over carb with a constant velocity carb
which is why you can hang a 2 inch bore SU on a 74 inch shovel that will never see 6000 rpm and it will run ok
put a 41mm Mik on it and it turns into a soggy dog begging for 36-38mm
there isn't enough cam, port, compression, rpm potential, or much of anything feeding those 200cc cylinders that will even use all of a 32mm VM
I haven't done the math on it since I don't know all the specs but off the hip
I'd say a pair of 28's would make one very crisp and snappy and a pair of 26's may be about right
that's about half the flow potential of the big 34's
and it would get good fuel mileage too
just for kicks and grins, what are the intake port dimensions and shape?
my guess is they are round...... tell me the bore size