It's a common belief that foods rich in estrogen (like Soya milk) increases breast size.
Eat Enough Estrogenic Foods. Estrogen is a potent female sex hormone and proper levels can lead to enlargement of breast tissue. Testosterone is the male hormone counterpart and diminishes breast growth. Avoid carb rich foods such as chips, crackers, white rice and baked goods to help minimize testosterone production. Excellent sources of natural estrogen include: pumpkin, garlic, red beans and split peas, soy foods eggplant, flax seeds, pumpkin and squash.
Try Soy. Soy products are high in isoflavones that can help the body increase its own estrogen levels and promote breast growth. Soy products are also rich in protein and can help build and repair damaged tissues in the body. Soy milk is available in a variety of flavors.
But the Straight Dope claims that this isn't true:
For women who do produce enough estrogen of their own, phytoestrogens actually decrease overall estrogen activity by competing with the homegrown estrogen for positions on estrogen receptor sites; when phytoestrogens latch onto these sites, they push aside the real estrogen and provide only a weaker version. In fact, that could be their real benefit, some experts think--by lowering the body's effective estrogen level, phytoestrogens may reduce a woman's chance of developing breast cancer. But here's the thing: if so, they'd likely make breasts smaller, not larger.
In short, whatever uses phytoestrogens may have, increasing breast size isn't one of them. Many breast-enlargement products contain only small amounts of phytoestrogens anyway, and none has been proven to work in double-blind laboratory tests.