If you compare the speed of the carburetor of a 30 mm carburetor with a 38 mm carburettor on one engine, it is certainly bigger in the 30 mm carburettor: the venturi effect, which allows the gasoline to be drawn from the float chamber, is stronger. The intake and atomization of the gasoline is therefore much better at low speeds with a smaller carburettor - but at high speeds it is not worse.
The cross-section of the carburetor must always be considered and not just the diameter specified. A 35 mm carburetor is twice as large as a 25 mm carburetor (25 mm = 490 qmm, 35 mm = 961 qmm). On Lambretta cylinders with delivered std. Cylinders and AF Clubman exhaust we already have a good 30 mm Mikuni. With a 27 mm but a little more at low speeds. Accordingly, we use 27.