How to increase breast size naturally?

No reliable method. It depends on what you mean by "naturally" but there are no non-surgical procedures that have been proven safe or effective.
See ABPS-certified. Plastic surgeon and inquire about breast implants. Fat grafting to the breasts is often marketed as "natural" breast augmentation, but not all of the transferred fat survives, and can cause scar tissue and calcifications that interfere with the mammographic diagnosis of breast cancer. Not worth it. Wait a few years when a few of women with fat transfer have cancer but miss or delay the diagnosis!
Weight gain. The only natural way is either to gain weight or become pregnant and then nurse. But when you deliver or stop nursing, they atrophy.