The reason that QA is done on radius is because the purpose of a ball bearing is to sit inside its housing to within a fairly tight tolerance. If the balls are

The reason that QA is done on radius is because the purpose of a ball bearing is to sit inside its housing to within a fairly tight tolerance. If the balls are out of spec, then potentially the thing the bearing is coupling will be out of alignment/won't rotate freely. The mass is less important. If you did QA on mass, you could easily calculate what mass would be required for a particular radius, but that would let misshapen bearings through. You could do QA on both mass and radius, but unless you have potential contaminants, the radius gives you the mass for free so there isn't much point.