thats the opinion of engine builder to differnt engine builder. Some like more case volume, some like less volume. Yamaha has always used fairly conservative sizing on all their engines as well as carb sizing where as other manufactures used 5mm bigger carbs on same size engine!
my personal opinion is you need to remember its a case reed engine, everything goes thru the case before its fed into the cylinders.
So having great big ported cylinders sitting on a stock case is like pouring liquid into a funnel backwards!