They’re not always. For instance, the default shifter model for OTK and Italkart are their 30mm versions. There are so many other factors involved (seat position, seat stiffness, axle, hubs, etc.) that you could either one to work as long as you’re not either a very skinny driver or a very large driver.
The 30mm will feel more lively. All things equal, it’s more flexible, so you’ll feel the track more, the inside rear will pick up faster, and come down faster. It also comes down to your style as well. If you’re used to muscling the kart into the corners, a 32mm might be better for you. Conversely, if you rely a little more on finesse, then the 30mm will be more appropriate.
I would discuss your needs directly with the CRG importer. As they get feedback both from their dealers and the factory, they should be the best candidate for accurate knowledge for their line.