I recently read the node Why do men like women's breasts? and discovered that a misconception existed -- large breasts have not been worshipped throughout history, as some would have you believe.
Marital, the Roman poet, wrote of the perfect breast as not overflowing one hand. Ancient Roman women, in fact, wore garments designed to support and suppress the bosom. Renaissance corsets squashed the breast in addition to the rest of the body. (belgand has pointed out to me that corsets tend to create cleavage as well) Europeans and Americans through the mid-1800s still preferred smaller, perky breasts with smooth contours.
Until recently, large breasts were seen as maternal, better suited to a wet nurse than to a beautiful woman. This may as well have been due to their impracticality -- a functional bra is a relatively modern invention.
Source: An Underground Education, by Richard Zacks. Truly badass well-researched book.