On thinking about this more, I will offer some ideas, and bear in mind, I'm no Jung.
Obsession with size almost seems to be harbored from the days of the school yard as teenagers. Sex as a teenager is something mysterious that is obtained by only the lucky and studly few. The guys who get it are revered by their peers, and the girls who give it are looked down upon. Well, this was in my experience as a teenager anyways, keep in mind, I went to an all boys Catholic high school.
I feel that somewhere, the idea is implanted in our heads that if you have a bigger wang, more women will desire you, and you're better able to please women in bed, which would equal being better and more sexed. The true stud as it were. Pretty much any guy who has the galls flock to him will always be highly regarded by his peers.
But here's something to consider, Beethoven was an incredible musician, it still didn't change the fact he was a total prick to know in person. Same kind of thing applies to guys who get all the girls and / or have a massive tool (yeah I'm trying to use as many penis nick names as I can
), if at the end of the day they're dickheads, then, well, they're dickheads. No one likes a dickhead (Americans don't use the term dickhead nearly enough hehe).
The interesting part is when I hear girls talk about guys they've been with and their respective sizes. It has always seemed to me like it's a "nice to have" feature, but certainly no deal breaker. But I have known girls who will use the size of a partners penis against him later, but only because they know it will cut his ego.
In a way, from what I've observed, the way women treat penis size is the way a lot of blokes I've come across treat breast size. We each have our own individual preference, but at the end of the day, it's no deal breaker, if our partner is Captain Wonderful, it's not all that important.