It is an ancient instinct. A woman with large breasts is a healthy and strong woman and is more likely to have healthy offspring. And they are less likely to die from disease. But with breast implants it's a bit useless.
I'm fine with small and flat chested even (hell yeah androgyny!) to average-large, but I really hate ridiculously large breasts. Especially on a ridiculously thin woman anime... (I'm sorry, but come on. That's not how it works. Usually.)