It's just genetic for the Apple Family girls to have large breasts. Granny Smith has them, Applejack has them, and now Apple Bloom has developed hers. But once her bust comes in in full and others begin treating her differently, she realizes that perhaps it's not as much of a "gift" as she once believed.
Rated T for partial nudity and discussion of breasts. There is no sexual content in this story, the tag is strictly because Moderators demanded it.