Radius (R) = Diameter (D)/2, thus D:R::d:r.
Double the "Radius" of a pipe & you get 4 Times the volume, but GUESS WHAT Double the Diameter of a Pipe & you Get "Four (4) times the volume.
As to a circle A (area) = PiR². = PiD²/4
Choice is strictly yours whether you prefer to work with radius or diameter.
You best go back to Math 101 if you think you can change the radius without correspondingly changing the Dia.
As the size of a shotgun bore is virtually always stated by Diameter I just work with that.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra