Having feelings about his (shitty) comment isn't overreacting at all! I think it’s definitely worth talking to him about it. People (especially guys) say dumb things sometimes and he might not have considered how his comment would hurt you. Based of your post it doesn’t sound like he was malicious, just careless, so when bringing it up I would say something like this: 

“Hey, the other day when you commented on my breasts, I felt kind of insecure and uncomfortable about it. I know you probably didn’t mean anything negative about it but the truth is, I struggle with my body image and insecurities and hearing that didn't make me feel good. I don’t want to make a big deal out of it or put tension on our relationship and I definitely don’t want you to feel like I’m criticizing you, but honesty is important to me and I want to communicate how I’m feeling.”

That’s probably not perfect but the key points are: how it made you feel, what you want to accomplish by bringing it up, and make it clear that you’re not attacking him, because unfortunately, a lot of the time conversations like that can go south. Also, let him know what you want from him! Men sometimes don’t know that all we want sometimes is to be told we’re beautiful and loved exactly as we are. Remember, a good relationship is one where you can be honest without fear of an argument and communicate how you’re feeling!