It's the engineering that's important. The bigger the flapper valve, the more water pressure pushing down on the thing, and the sooner it fails unless you reinforce it lots more, which means extra costs, either for the first one, or when you have to periodically (more often than a 'normal' one) replace it. When everyone went from the old monster flushes to the 3.5g ones, they often just reduced the amount of water flow without changing the design. THat was still enough to work okay in some cases. Then, when the requirements dropped to 1.6g, they did the same thing and those that didn't re-engineer things ended up with toilets that were real dogs. Terry said he had a tour of the Kohler factory when this was discussed...they went to a 4" for the bragging rights, not because it was necessary. Kohler tends to have the not invented here disease and likes to do things their own way, regardless of whether it's a good idea or not!