One of the things that has always fascinated me about manga is its short-hand symbolism. Betty Edwards in “Drawing on the Right Side of the Brain” speaks about children being encouraged to draw symbols instead of reality (a “box” house for example) in early art classes and thereafter lose their ability to naturally “see” what’s really there. This skill needs to be regained in order to draw accurately. Interestingly, the reason that manga works so well is because of its symbols and in many cases its exaggerated reality (at least in reference to the characters.) Manga symbols are used as an additional language that, once understood, remains constant, adding another layer of understanding to the reader. Although western comics have a symbolic language too, I find that it is much less developed, or at least less refined and consistent.
So what about manga mouths? More specifically shojo mouths? Shojo is my personal manga style of choice, and is defined by Wikipedia as “[manga] marketed to a female audience roughly between the ages of 10 and 18… Shōjo manga covers many subjects in a variety of narrative and graphic styles, from historical drama to science fiction often with a strong focus on human and romantic relationships and emotions. Strictly speaking, shōjo manga does not comprise a style or a genre per se, but rather indicates a target demographic.”
So we’re talking about comics that are specifically targeted to girls for the purpose of creating comic-style drama around relationships of some sort (friend, love, family, etc.). This requires a special language of symbols designed to heighten emotions. The mouth is frequently exaggerated in order to suggest particular emotions or sounds (not necessarily associated with words.) But with the caveat that women and girls (especially) are supposed to have small, delicate mouths so that a larger mouth is meant to indicate a more comic reaction.
Here are some examples: