Between you and me, uh, Masters and Johnson's information up to this point has not been circumspect to all aspects of endowement measurements, since I recently had to bone up on some material for a term paper on genetics, and reluctantly only found in the library, a book by M&J going on nearly forty years old.
It would appear that the only pertinent 411 they had was the simple mechanics of penile size, and function.
What's more, they implied with rather righteous ascertainty that the size you have had is the one you're stuck with, and nothing would change it.
I'm 10.5x7.5 in bed, and going on 8.5 flaccid; that means that I'm significantly 2.5 inches longer soft than the "average guy" is hard.