Flat chested? Is this due to a hormonal imbalance?

Usually. Controlled by your genetics. Breast size is extremely variable and not considered an abnormality or deficit because your breasts are smaller than another woman's. Breast size has nothing to do with femininity or sexiness. It's a western culture thing about size.

Related Questions

I'm 20 yet doesn't fit any bra size. I'm flat chested, is this hormonal imbalance? Is there a solution?

Maybe. You should see a fertility specialists to get a more full assessment. It is important to understand your full puberty before knowing if there is something abnormal going on. There may be some treatments available but they usually don't work as well at 20 compared top 15. But time is of the essence to be evaluated.