I'm going to tell my girlfriend I would find her a little less attractive if she got a boob job
Why would you tell your girlfriend that she was less attractive?
I don't care about the size
Well, I think it is very rude to dictate someone what they should do with their body. It is impolite and encroaches on their body sovereignty. You should only do that if they ask for your opinion.
Your advice is not helping me, I don't care about what you said.
I'm just trying to help you out. You don't control your girlfriend so you shouldn't be rude to her. She can do what she wants and you should support that.