I understand her pain. I was flat-chested. I wore a training bra (the not-even-a-size-A size) from the girls' section until I was about 20 and started putting on weight (very skinny until then due to being very active, met my husband and stopped the activities). Kids were cruel, especially the boys. The girls weren't so bad. This is backwards from what I hear. You need to find out WHY she is embarrassed. Is it because her friends are "always" talking about it? Are the boys calling her a "carpenter's dream"? (this was the one they called me most - for the punchline anyone can email me). I also had the added problem of 2 younger sisters who developed in middle school. They were both a size C before 8th grade and the older of the 2 never let me forget it. She would introduce me as her brother since I had short hair and a "boy's body", as she put it. If it is just her own opinion of herself, then she needs to learn to stop worrying about what she can't change.
Having larger breasts will only give her problems in the opposite area. My sisters got called worse names (slut, boys would make lewd comments, etc.), simply for being big-breasted. They hadn't even started dating, yet.
Yes, it can be hard for her even without the media's influence.
Remind your daughter that she has a lot more freedom since she doesn't have to wear a bra. She can wear almost anything without worrying if the buttons are going to close. I got a LOT of cute clothes that my sister couldn't wear simply because her chest was too big! At 16, I always got the cuter swimsuit because without the large breasts I could get away with less material without my parents feeling it looked too sexy. She had to buy from the adult swimsuits and she wasn't allowed the hot ones because she was only 13, just the one-pieces like our aunts wore. I could do SO MANY MORE activities since I didn't have the bouncing breasts. Both of my sisters now have HUGE breasts: one is in a DD and the other is in a EEE. Thankfully I am only a C. My sisters are both considering breast reduction surgery because their backs just can't take the wait anymore.
When she starts a pity party about how small she is, remind her of what that allows her to do. She can jump on a trampoline without having to wear special bras first! She needs to learn that the kids who are saying things to her are only trying to make her feel bad so they can feel better. As hard as it is to learn, people can only make us feel bad if we let them.
If she is still flat in her twenties, then she can see a plastic surgeon, but I wouldn't do it yet since she is still in puberty.