Dude the answer is really simple from evolutionary biology.
Girls get big tits to feed children.
When you are a child you want to be loved and love a mommy with big tits
When you get older, you want to turn girls who are not mommies into mommies, so you're attracted to small tits because they haven't had babies yet
Pretty soon you'll start checking out asses to determine whether they have enough energy stored up in their fat deposits to create a baby over 9 months.
Meanwhile those girls are going to be checking out your upper body aka punching muscles because that's how they identify strong protective males to father their young
I really miss being a little kid and getting flustered by cleavage, but now hips and butts are all that excite me.