^ I would venture to say it is the same way you men worry about penis size. For women, fuller or larger breasts means you are perceived as more womanly or feminine or even a better lay. Smaller chested girls can be looked at as boyish or youthful. Breast size seems to define how womanly we are.
True, it shouldn't matter. I think most of the onset of breast size insecurity for girls is during puberty and through high school. We see other girls blossoming, wearing tight t-shirts with their cleavage, and boys stare. And those of us with smaller breasts don't necessarily get that kind of attention. However, from my expeirience, all the girls who grew large breasts in high school are now chubby or over-weight now that the rest of their bodies have caught up with them. For that, I am greatful to be a naturally thin gal with a B cup.
I have considered having my breasts augemented many times and I still do when I see models or actresses or just women in movies with beautiful, full breasts. Not that I want mine to be freaky Pamela Anderson-sized boobs. But a full C cup would be preferred. However, from my understanding women with implants can't breast feed their children and that is something I want to do when I have kids. Also, it's terribly expensive and there can be complications and health risks. That's not something I'm interested in taking a chance on just to fill out my bikini top better.
Luckily, my boyfriend is an ass-legs man like you, Herve. I would say my legs (and lips and hair) are my best physical attribute, but I unfortunately live in a place where it is cloudy and rainy for 200+ days out of the year. My boyfriend loves my breasts and tells me he doesn't want me to change them. I'm not nearly as insecure as I was about not having big breasts and I'm fairly happy with them as they are. But I do still get envious of the more endowed women.