First Cervical spondylosis is a decrease interdiscal space not a discease .Normally people refer any cervical pain to cervical spondylosis.So this word is used by layman.You always need a thin pillow like 3-4 cm height soft pillow to compensate normal cervical curve.
The rationale is to maintain a neutral position of cervical spine, especially in a degenerating spine, so as to prevent stretch induced micro injury to the cord. The neutral position is a lordosis, which can be maintained only by using a small/ thin pillow. If no pillow is used, spine will have to go in for hyperextension, which is not physiological. Also, if you are in a habit of lying by your side, the pillow need to be as thick as your shoulder to maintain neutral position.