Why are Japanese Cups so Small?

When I was in Japan, I noticed the biggest difference between the portion sizes compared to America was not the food, but rather the drinks. I drink a lot of water with my meals, so it was embarrassing to constantly call the waiter over for a refill of my tiny cup. I usually drink cold water as well, which was even worse as some places had hot water dispensers for tea at the table, but not cold water. (This was during summer and a lot of the food I was introduced to was hot/spicy.)

Surprisingly, the portion sizes for food were similar to what I get in the US, but the cups were much smaller compared to the large sizes you get in the US. I wouldn’t be surprised if they were smaller, but it was to the point where only one or two drinks and the cup would be empty. I was also surprised that my Japanese friends hardly needed to drink anything while eating, even if it was heavy or dense like かつどん. Does anybody have an explanation or can relate to this or is it just me?

https://www.reddit.com/r/japan/comments/yg3ixk/why_are_japanese_cups_so_small/