BOTOX® is almost always used to relax various muscles [and rarely used to completely paralyse certain muscles].
Certain individuals who suffer from a condition called torticollis ["neck twisting"] benefit from relaxation of the sternocleidomastoid [and sometimes other muscles] after treatment with BOTOX®. After treatment with BOTOX® the sternocleidomastoid might not contract so hard, and so would not stand out so much, but it would be unlikely to shrink.
The same effect can be seen when the flat muscles ["platysmal bands"] of the anterior neck are relaxed with BOTOX®: they do not jump out as much when the individual speaks or makes certain movements, so the neck looks smoother and more attractive, but the muscles do not actually shrink.
In cases where there is excessive growth of a muscle [for example, the masseter muscle, in particular in people with bruxism who grind their teeth at night] find that relaxation of the masseter with BOTOX® controls both their bruxism and the associated tempormandibular joint ["TMJ"] pain, and are pleased that the size of their masseters gradually comes down to normal, so that they do not have a chipmunk-like appearance.