I have a rescued bike with rear Maillard Helico Matic hub. I decided to replace the ball bearings and removed the old ones. Cages, balls, and cones looked ok. I measured old balls and they were cca. 3,7mm in diameter. I looked around on the intewerbs and found this site with the picture below. The hub on the picture looks exactly like mine. Well, the decal has a bit different graphics but by eyeballing everything else is identical. The picture says that this hub takes 5/32" ball bearings, which is bigger than my measured 3,7 mm by about 0,2 mm. Is it possible for ball bearings to wear this much?
When bearings wear down, they tend to wear unevenly. They pit and chip and generally come out of round. The races tend to look equally rough if they're the same age as the bearings. If all is looking to be in fairly good shape, I think it's far more likely that someone has previously serviced the hub and used slightly smaller bearings.