How can I maximize my rabbit litter size? ?

I have rabbits that I breed and sell for 4H and FFA. My rabbit usually only has 3-4 babies. I hear stories about people who have rabbits having 8 babies and my rabbit is producing less than half that. She is a full grown rabbit too. Average size mix breed. If I breed her with the buck more that day will it make her have a larger litter? Or is it just their genetic make-up?