Lingerie manufacturer Frederick's of Hollywood reports that in 1996, the average size of bras they sold was 34B. Currently, the average is a 36C. Now, granted those who purchase lingerie from Frederick's are not statistically representative sampling of American women. Nonetheless, this change is extremely interesting.
Similar data about the increase in average breast size comes from the UK, where busts grew from 36B to 36C between 1997 and 1999. According to The Penguin Atlas of Human Sexual Behavior, breasts across Asia grew from 34A to 34C between 1980 and 2000. It seems that the average breast size is increasing around the world.