I think it takes time to get used to change. Boobs only started being part of the equation for a short time for you, so it's harder not to notice them. But I don't think cis girls aren't aware of their boobs. In fact, they're aware of them all the time. Society bombs them with beauty and body pressure that is undeniably. The ideal girl must be super thin, but have huge boobs and ass, which is biologically almost impossible. And heteronormative guys in general feel entitled to look at boobs, and there's lots of shaming. Both from guys who comment on girls' size and other girls as well. So I wouldn't say they are always aware of their boobs, since they've had them for most of their life and are just used to them, but the attention they get or don't get from other people reminds them of how powerful boobies can be. Like any other woman's body characteristic actually, since women's bodies are so much more objectified than men's.