Hi. Welcome. An interesting question from a woman.
Here's my answer.
Alot of what is prized by men in women, and vice versa, is learned from parents or other role models we have in life. If you grow up around male role models that make comments like "Whoa--look at the melons on HER!", you're probably going to be more inclined to think that curvy women are somehow more desirable. That's a silly thing when you think about it, but we learn alot of social behaviors from our parents/family.
A second theory is what I refer to as the Howard Hughes Plot. In 1943, Hughes saw Jane Russell in a dentist's office and fell in lust with her. He developed a special brassiere for her considerable bust and then cast her in a film he was making, called The Outlaw. The film, after it got released, caused a catastrophic stir, and accomplished Hughes' objective of making Jane's breasts the talk of the country. What followed, in the 1950's, was a sexual revolution of sorts. Women wore padded bras, bullet bras and falsies in an attempt to be "more womanly", "more feminine", becuase the screen starlets of the time were women like Marilyn Monroe, Jayne Mansfield and Mamie van Doren, and all one had to do to appreciate their....ahem...charms...was to look at their pneumatic profiles.
To sum up, it is definitely a cultural thing to some extent...the 1950's was a decade of big bosoms, the 1960's-1970's was a decade of small ones. And on it goes.
As for me, personally, when I was about 16 or so, I got to see The Girl Can't Help It on television. It stars Jayne Mansfield. And her 40 and one half inch bust. And nineteen inch waist. And a figure that literally stops traffic. And from that moment, I was hooked.
What is it about large breasts? I dunno. That would take about a billion shrinks to figure out. I like the mystery created by a huge cleavage. I like the way big breasts feel to the touch--warm, soft, inviting. I like the way they move, the jiggles and bounces they create.
Was that Too Much Information?