The Japanese mostly eat sticky rice, which is very easy to eat and "clean up" even with a chopstick.

The Indian subcontinent eat long-grain Basmati or similar rice which fluff up into individual grains on the plate. It doesn't make sense to individually pick out single leftover grains.

In nearly every culture is the idea of "Annapurna" or the god of food, and wasting food is generally frowned upon and considered bad table manners. I've been scolded plenty of times as a child for not cleaning up my plate in Nepal.

I wouldn't attribute it to small bowls at least. The Japanese instilling good virtues into their children almost institutionally perhaps plays some part in it, but also some of it is just physics.

Having had grandparents live through WWII (or any other war to be fair) also helps instill this attitude. I can barely imagine what kind of famines they had to endure.