Sphericity and size accuracy are two different things. Sphericity is how much the surface deviates from a perfect sphere and it independent of the size of the sphere. In other words, how smooth the surface of the bearing is.
I think you meant that sphericity is a measure of roundness. I.e. you could have a rough surface or smooth surface bearing but could have equal sphericity. And I am just nitpicking but I would say "how much the shape (not surface) deviates from a perfect sphere". A surface is 2-D, a sphere is 3-D. Excellent definitions though.