First, most guys love boobs. They are uniquely female and very desirable. Guys are aroused visually more than anything. And part of the reason guys like boobs so much is genetic: it is an indicator that a woman is sexually mature and can bear children. Our instincts direct us to "spread our seed", which means having sex with lots of women, which increases our chance of passing along our genes. It also means choosing women who are likeliest to bear healthy children and to take care of them. Breasts are an obvious indicator of this.
So, guys are attracted to breasts and enjoy looking at them. Not all guys to the same degree; some guys are more into butts or legs or feet or whatever, but they still like breasts.
And larger breasts are more noticable, more obvious, and they are somewhat more rare, so they tend to stand out more in a person's notice, kind of like a red Ferarri in a sea of beige Hondas.
And of course, "boob guys" will notice them, and "big boob guys" will even moreso. In fact, the guys who love big boobs will often go crazy and act like idiots around them. That's not all guys, but again, you notice the exceptions more than the average.