2.5lbs. the heaviest in the world was about 40lbs. each. *correction*
The world's heaviest breasts: 52 pounds?

In one recorded instance ("The Sexual Anatomy of Woman" by W. F. Benedict), a girl of fourteen had breasts weighing sixteen pounds; and in another case a woman of thirty had breasts weighing fifty-two pounds. As with the male sexual organs, breasts are sometimes grossly enlarged by disease or some other unwelcome cause. For instance some women have hyper-trophied breasts, sagging vastly and of quite giant proportions. An Abyssinian woman with this condition is shown by Mantegazza in his book, "The Sexual Relations of Mankind".

From The Illustrated Book of Sexual Records. 1974, 1982, 1997-2001 G.L. Simons