Although Botox could theoretically reduce the size of the sternocleidomastoid muscle, it is not advised, since this muscle is important for head control and movement. You may want to consult with a neurologist to discuss possible reasons for the enlargement of this muscle.
I have noticed an increase recently in people asking about using Botox to shrink normal body muscles because they don't like the look of the muscle. Botox paralyzes muscles, and the muscles of the body are used for structural support, normal movement, and safety (reflexes require normal muscle function to protect you from unexpected injury or danger). Botox is NOT to be used to shrink otherwise normal body muscles. If there is a medical problem with muscle spasm, see a neurologist to get the appropriate amount of MEDICALLY NECESSARY but safe Botox. Do not go to any non-MD providers for muscle issues with Botox. That would be extremely dangerous.