Big breasted anime characters

That doesn't make a lot of sense, but I'm going to guess that you are talking about after having relations, a woman's breasts seem to grow. And if you are, it might be because of a release of hormones that causes them to swell. The same thing happens during their period.

Well I never noticed any change in size after just playing with them. But then again, my mind was usually focused on something besides the size of her breasts.