While all vertebrae have approximately the same shape, the lumbar bones are the largest. This is because they carry a (larger) load transmitted from the top of the head down to the low spine.
Another reason why the lumbar bones are larger than the cervical and thoracic vertebrae is that they must be big and strong to support the muscles and bones during movement. The lumbar area of the spine is the physical center of the body. This area is the hub for many basic activities, including balance and the generation of locomotion (walking and running, etc.) The demands on the lumbar area are great.