Typical Japanese will devour their small rice bowl until there's none of rice grain is left over, since they're taught from the very young age not to waste food.

Most of other Asian nations will not eat their rice until it's completely finished. Even with their most delicious biryani dish there're always many rice grains left in the plate. I think the small bowl make it much easier to completely consume the rice unlike the big bowl or plate.

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.

Wasting food is faux pas in many a eastern cultures, including South Asian & Middle Eastern.