Even if, hypothetically, looks counted for EVERYTHING (and, of course, they DON'T), the degree of how attractive a woman's breasts are, to me, is AT MOST 2 or 3 percent of her overall looks rating (as, to me, having beautiful breasts are FAR less important than having a beautiful face, and quite a bit less important than having beautiful: stomach, hands, ass, legs, hair, and complexion).
And the SIZE of her breasts is less important still.
Just as many beautiful people look in the mirror, and miss all of the beauty, and instead see only the one tiny zit that nobody else even notices, many men and women, in my opinion, unnecessarily obsess over their "less than perfect" penis/breasts/etc.
Short answer:
"FAR less important than anyone to whom it would occur to start a thread asking this question would imagine."