Correct - the bit is fluid so doesn't lie straight in the mouth. You generally go up 1/2" on your regular size, this is most important on the loose ring, but you can get away with only 1/4" bigger if its a fixed cheek.
Taken almost word-for-word from Neue Schule's "Enlightened Bitting"
Waterfords are normally worn ¼"- ½" longer than a traditional mouthpiece to curl around the lips and so maximize the effect. A Waterford Pelham is not worn longer than normal otherwise there wouldn't be any curb action.