While page Skill has "Largest bone available" and "Smallest bone unavailable" written, I don't see they are like what in game at all. That is, both largest and smallest bones are availabe for fully-grown and non-fully-grown Bone skills. I tested them with two game windows, but I can't tell any difference (e.g. speed, angle, bone probabilities) except the damage. Was it a bug, or just designed as "no improvement other than damage"?
Before final release, this statement was correct. As of now, it seems both the largest and smallest is present, no matter if it's mastered or not. There might be changes to the probability of small/large bones somewhere from the original design.