M.B.
Hi D.,
I had mine done a few years ago and my explanation was the total truth approach. I was unhappy with the size of my breasts (barely an A) and I was choosing to have them increased to make me feel better. I was VERY uncomfortable about hardly having any breasts and thought this would be the right choice for me. I think you should just give her your reasons and let her know if she has questions to come to you. I wouldn't make it a big secret as that is alot of pressure for a kid. So what if she says something people always have something to talk about!
Best Wishes on your talk!