The general idea is that a cup of rice is the main portion of the meal, and the rest of the meal is add ons (mostly veggies with a bit of meat). If the rice is just a side to the entree, then yeah, you're going to be eating less of it.
That's the idea but as someone who have been in Asia all my life rarely do people eat the entire cup per meal. 0.5-0.7 cup per person is more common this days for standard meal.
