Thank you for your query at DoctorSpring.com
I can understand your concerns. Many educated parents with keen observation skills do present with the exact same problem.
The answer to this observation is that it is a natural variation. In our body all bony ends start of as cartilages (soft structures, a little bit similar to our ears) in what are known as ossification centres. Then as the child grows up gradually, these cartilages are converted to bones by the body. This process is also influenced by local shear and stress. So as the child grows up and starts using one dominant arm, the mechanical stress to each claviculae end differs. In view of this, asymmetry of bony ends of clavicle is common but never problematic. So I would request you not to get concerned about it.
If this asymmetry is associated with any of the following, then you need to get back to a physician:
1. Increasing size of one claviculae end - can suggest a bone tumor, joint infection, etc
2. Painful movements of the arm and shoulder or restriction of joint movements
3. Local tenderness over clavicle bone
4. Neck deformities or associated asymmetry in neck muscles ( torticollis)
5. Other bony swellings anywhere else in the body.
I hope my explanation has helped you understand the problem better.
Dr. Saptharishi L G