You shouldn't express your opinion unless she asks you and if she asks you tell her your opinion once and never bring it up, again. or contradicted her choice. As far as aesthetics go she should listen to her friends, and medical decisions she should be discussing with a health care practitioner.
If she wants to talk about her large breast issues with you be supportive (he he he)
I don't think teenage boys should be playing a major role in making life decisions for a teenage girls they are dating, even if you have good intentions, tremendous knowledge and wisdom and great insights.
I am just silly like that.