Verified by 
In his fascinating book Cultural Psychology, Steven Heine discusses studies that have investigated cultural differences in how people interpret facial expressions of emotion.
In some parts of the world—the Middle East, for example—people are very expressive. They tend to show everything they're feeling. With their body, their hands, their face. Nothing is concealed, nothing is disguised. In fact, what is presented may be a deliberate exaggeration of what one actually feels.
In other parts of the world—Japan, for example—people often conceal their emotions by placing a hand over the lower half of their face or showing a neutral facial expression.
The mouth often conveys a lot of information about how someone feels, so it makes sense to focus our attention on a person's mouth when trying to read their emotions. But it's also relatively easy for us to control the muscles around our mouth and conceal our true feelings.
The muscles around the eyes, however, are more difficult to control. In societies where people often wish to conceal their true feelings, it makes sense to focus our attention on a person's eyes. They are, after all, windows to the soul.
In 2007, psychologists Masaki Yuki, William Maddux, and Takahiko Masuda reported the results of a clever study in which participants made judgments about people's facial expressions. They constructed a special set of photos in which the top half of each face showed a different emotional expression than the bottom half. American and Japanese participants looked at each photo and had to decide what emotion the person in the photo was expressing.
The Americans were influenced mostly by the bottom half of the photo. They looked at the mouths and relied on that information to "read" the person's feelings.
The Japanese, however, were influenced mostly by the top half of the photo. They looked at the eyes and relied on that information.
In Japan, the eyes reveal more—and the mouth reveals less—about what a person is feeling. Maybe this cultural difference in reading facial expressions can explain why Japanese anime characters typically have huge eyes and tiny mouths.
References
Heine, S. J. (2016). Cultural Psychology. New York, NY: W. W. Norton & Company.
Yuki, M., Maddux, W. W., & Masuda, T. (2007). Are the windows to the soul the same in the East and West? Cultural differences in using the eyes and mouth as cues to recognize emotions in Japan and the United States. Journal of Experimental Social Psychology, 43, 303-311.
