If I were flat chested or had a National Geographic sag, I'd get a modest boob job.
I was lucky to be genetically predisposed to having nice breasts. They are also proportionally sized to my body. Other parts of my body I'm not necessarily thrilled with, but the boobs are fine.
Kate Hudson got a very modest boob job, though I think she has officially been denying it. It's an improvement. She should admit to having plastic surgery, there's no shame unless it was done badly.
Megan Fox had a boob job--her "before" breasts were too small for her body . . . they also seemed to do the National Geographic thing. Her after ones aren't perfectly shaped but they are much better than what she started with. All of Megan Fox's plastic surgeries have been vast improvements: the smaller nose, even the bigger lips.
This model from American Apparel is what I would call "nice boobs disproportionate/too small for her body".
I think that a significant percentage of women with flat chests were anorexic during puberty. I have a theory that being anorexic in puberty can affect hormones, stunting breast development that would normally occur.