I found a delightful breast expansion story at the BE Story Club called "Accidental". Written by "Gunslinger", "Accidental" is about four college women who, following an automobile accident, encounter some strange chemical storage drums in the woods. In the hours following the crash, each discovers that her breasts are slowly (or rapidly, depending on your time perspective) growing larger. Much, much larger!
Although flat-chested Jen is at first delighted to find her formerly small boobs growing to spectacular proportions, even she begins to panic when they just keep growing! And the other three already had decent-sized or big breasts already! How huge will their tits grow? You'll have to read the story to find out!