I'm sure that there are plenty of factors that contribute to the size of roots. Here is a picture of one that stumped me a little bit. Its hard to tell now that its dried, but this root is 47 years old. What you see is how it came out of the ground- there were no root hairs or any other parts, as it was dug very carefully. About a foot away form this one was a very nice 8 year old root that was very nice size. This small root put up a decent 3 prong plant that had 6 berries ( I wish I would have taken a picture of the top). In close proximity to this plant were a dozen or so other plants all of normal size with respect to age. So ideas as to why this is so small?? Animal damage?
About an inch from the root at the turn there seems to be a scar. That is about where another bulb might have grown and something happened to it. I have found some roots in sandy clay that part of a root has root rot, and probably would not last long. That seems to be a small root for the size of the age rings.