First of all, some women may have unpleasant memories about the teasing that they got when they were younger but as they got older, they were happy with their body. That is why it is important to avoid telling them that they should change it in some way or another, such as getting implant surgery or even wearing a padded bra. Before making a comment consider this, if you can’t think of any thing to say on the subject that is flattering, avoid the subject altogether (Source: Lula Lu 34a bras).
You should also not make them feel like what they have is of lower quality than what you have. Even if you are large busted, don’t draw attention to it by telling them that you would never be able to wear shirts like the one they were wearing. And above all never tell them that there is no need for them to wear a bra because of their small breast size. Just keep in mind it really isn’t any of your business, so avoid making those comments.