When those ISAF classes were set up, it was just a way of loosely categorizing all the catamarans so they had a way of racing together, boat for boat. But very soon, early 1960's in the U.S., the Portsmouth system was established for the cats, so the ABC classes became irrelevant, except as a way of generally dividing up groups of boats in a regatta.

I think the ABC classes were based more on sail area than on boat size. That is why the Hobie 16 limited its sail area to B-Class size, which was the same as the Tornado.

They weren't like formula classes where peope build to the specs. It was kind of the other way around, where the classes were designated based upon the boats existing at that time.