Lol, did you read the "most cases" part?Lol, 1 stack for 40 provinces works in western Europe, Japan, and India, maybe. Try that in northern Asia, central Africa, and a few other areas sometime and you'll see a different story. When it takes 2-3 months to move 1 province every bit of defensiveness you can find is the order of the day if you don't want to see rebel controlled provinces.
Or do you really think Siberia and Western africa are common case? Maybe you`r huge empire should waste it`s NIs to conviniently pacify a smalish part of it?