How does ocp stop my ovulation but cause have shorter, lighter periods?

Daily estrogn & prog: Daily estrogen stops the brain from sending signal to the ovary to stimulate ovulation. Daily estrogen would cause the uterine lining to grow thicker. Daily Progesterone was added to supress the lining from growing. This gives you the added benefit of having light periods. Sometimes the balance of Progesterone to estrogen is so high that it supresses periods completely. If needed change pills.

Effects on pituitary: Birth control pills reduce pituitary hormones that would ordinarily trigger ovulation, so pregnancy is much less likely. In addition, the pills limit development of the uterine lining, providing additional benefits of reduced flow and additional protection against pregnancy.