I want to let my girlfriend know that her boobs are starting to look like rubber balls
Why would you say that to her?
I don't feel like I want to cuddle with ball-sack boobs.
You sound like you're deeply turned off by your girlfriend's body. Is this really a deal-breaker for you? If so, then you shouldn't just make crude comments about her body, and you should be honest with her about the fact that you don't see a future together.
Anyway who wants to cuddle with ball-sack boobs?
I would think that you would care enough about your girlfriend to look past her less-than-perfect body, but if you aren't happy with her, then it's time to move on. If you treat her badly, that would be worse than just being honest about your desire to be with someone else.