His specific form of dwarfism is defined by having a normally proportioned head and torso, but shorter limbs due to a defect in bone formation. So not just head and throat, but chest and lungs. Compare with Herve Villachez, who had an endocrine disorder that made his entire body smaller, and who had a notably high pitched voice.
Of course, physical build isn’t the only factor in determining what a person’s voice sounds like, but it’s definitely a major one.