There is no good answer for this question. The most possible explanation after all my research is below:
1) In Japan, tea is revered as one of the most important beverages in the world. Tea ceremonies (cha-sabuki) were once an important part of Japanese culture, and people would gather together to drink tea and discuss politics or business.
As a result, Japanese teacups are designed specifically for drinking tea. They're not meant to hold other drinks or food - only hot water and a loose-leaf bench (tea). This design allows everyone at the ceremony to have equal access to the cup, which encourages communal drinking.
So why do Japanese cups tend to be so small? It's all about convenience!
2) Japanese tea cups are famously small, and there is a good reason for it. In Japan, tea is seen as an important part of daily life, and properly drinking tea is considered very important. Japanese teacups are designed to hold less liquid so that they can savor each sip.
This tradition has carried over into other parts of the world, and smaller cups of tea are now popular everywhere, from Europe to North America. Some people find them more convenient because they don't have to wait long for their drink to cool down before taking a sip.
So, what's your preference? Smaller or larger Japanese teacups?