The Small subgroup moves and rotates with the axis, but all of its faces keep their shape and orientation with respect to each other. Maybe an example will help. Think of a robotoid arm with no fingers, no wrist. The child axis sits at the center of the elbow, with its Z axis pointing at what in a more highly evolved robotoid would be the hand. You need two subgroups for each bone.
Lets start with the forearm.
The Big subgroup for the forearm will include all of the forearm and just a little of the upper arm above the elbow, enough to allow the joint to stretch to maintain a smooth connection. The Small subgroup will contain most of the forearm, up to just below the elbow. The faces around the elbow which are in the Big subgroup but are not in the Small subgroup will be the ones which stretch and deform to allow the joint to connect smoothly. On the upper arm, the Big subgroup will contain all of the forearm, all of the upper arm, and a little of the shoulder it is attached to. The Small subgroup contains all of the forearm, and most of the upper arm up to just below the shoulder.