Unfortunately, having the breasts grow at different rates is a normal part of development. During puberty, breast growth usually begins first (before menstruation or body hair). Therefore, most people have 4 years or so of breast development during normal puberty. The breasts grow under the influence of sex hormones in the body and, unfortunately, it is quite common for one breast to be more sensitive to the hormones that the other and grow more rapidly.
Almost always, there is catch up growth of the smaller breast with time, so that by the time puberty ends both breasts should be about the same size (although most adult women still have small differences between the size of their breasts).
There is not much that can be done medically about this difference in breast size during puberty. Once puberty stops, if there is still a large difference in breast size, cosmetic surgery
can be done on the smaller breast to make enlarge it and make it look more like the other breast. This is usually not done unless there is a full cup size difference.
For now, the best thing to do is to encourage your daughter that this big difference in breast size will slowly get better.