It is possible for the scar from childhood open heart surgery constrict breast tissue development when placed across breast tissue, occasionally resulting in asymmetry. However, it sounds like your daughter simply has small breast development simply as a result of natural breast tissue responsiveness to hormones. The obvious solution would be breast augmentation, a safe and effective way of enhancing breast size. I would suggest a consultation with a plastic surgeon to evaluate her condition, assuming this a route your daughter is interested in pursuing.
At what age do you feel normal development is complete and augementation considered as an option .