The herb fenugreek (Trigonella foenum-graecum) has been used traditionally to treat menopausal symptoms and digestive problems and to induce childbirth. It is most widely used today to boost breast milk production and, in much larger doses, as a source of dietary fiber.
I discussed your question with Tieraona Low Dog, M.D., director of The Fellowship at the Arizona Center for Integrative Medicine, and an expert on botanical medicine. She is familiar with the use of fenugreek in supplements that purport to enhance breast size and thinks this is based on folklore. She tells me that there are no published scientific studies in women to support this claim.
Fenugreek contains diosgenin, a compound used as the raw material to produce the hormone progesterone in laboratories, but Dr. Low Dog said that the human body is not capable of converting diosgenin to progesterone. It is unlikely that you'll suffer any adverse effects when using daily doses of three grams or less, but Dr. Low Dog doubts that fenugreek will make any difference in your breast size.
I've been asked before about the use of other herbs for breast enhancement. I know of none that work. Breast size is determined genetically and depends on the ratio of fat to other breast tissue. Other than the temporary enlargement that occurs during pregnancy and nursing, the only way to increase the size your breasts naturally is to put on weight - the amount of fat in the breasts varies as you gain or lose while other breast tissue remains constant. While I wouldn't recommend gaining weight as a means of breast enlargement (unless you are now underweight), exercise to build and strengthen your pectoral muscles can make breasts appear to be larger.
Otherwise, I can only urge you to accept your body as it is and be thankful that you're healthy.
Andrew Weil, M.D.