A concept typically found in hentai, but not limited to it, breasts may suddenly begin to grow larger in size with little or no explanation.
Breast Expansion is the idea of unnaturally rapid breast growth in women, in some cases going from a flat chest to very big boobs in a matter of seconds. In entertainment media, this is usually used for either comical or sexual purposes or in some cases is simply part of a transformation and is given much less focus.
Usually used as comic relief in various anime and manga franchises, Breast Expansion typically happens to one or more unsuspecting female characters with little or no warning, usually resulting in hilarity at the expense of the characters involved.
Depending on the anime/manga, the expansion of the female character in question's breasts can have several different results, anywhere from using their new-found bust size in order to attract the attention of male characters to the totally insane result of being carried away high into the skies due to their "ballooning" busts.
During accelerated breast growth, characters generally react in one of two ways, either freaking out in a most spectacular fashion, or finding the growth stimulating and pleasurable. For the most part, this depends on whether the concept is being used comically or sexually.
Breast Expansion also occurs in a number of hentai franchises, but it typically happens for different reasons and results in different outcomes when it does take place. Since hentai is meant for a mature audience, the instances of Breast Expansion taking place usually occur in one of the following ways: