I'm not sure how to handle this. My gf is naturally very well endowed. I don't make a big deal about it, but a lot of my friends do. I'm not sure what to say so I just smile. Is that ok?
You could go the whole "it's so disrespectful" route but I doubt it would get you anywhere. You could get upset and tell them to STFU. You could ask them nicely to knock it off. Or you could ignore it. The problem with the last one and just smiling is it doesn't stop their behavior. Ask them nicely once. If it happens again, it's ok to confront them. Just don't have it escalate to something physical.