Final answer:
The vertebrae increase in size from the cervical region down to the lumbar region for adaptation to different movements and weight-bearing.
Step-by-step explanation:
The functional purpose of the vertebrae increasing in size from the cervical region down to the lumbar region is adaptation to different movements and weight-bearing. The vertebral column, with its S shape, acts as a shock absorber, distributing the weight of the body and protecting the spine from breaking. This adaptation allows for flexibility and support of the body during various activities and movements.