So what are you expected to do with the last few sauce-soaked grains of rice that would at best be able to be plucked grain by grain from the bowl, and even then would likely slip from between the tips of the chopsticks? Just leave them in the bowl?
So at the same time it is considered poor taste to take more than you can eat, it is also considered poor form to offer a guest anything less than more than they can eat. This also shows up when people rate restaurants by the serving size.