Based on the idea in Weber's Law in which the just-noticeable difference in a stimulus is proportional to the magnitude of the stimulus, what is an advantage of having motor units of different sizes recruited in an orderly manner? If there were a small weight, for example, then small motor units would be recruited.
Answer
If you add a small weight, you want to recruit a few more muscles, or small motor units. If you recruited large motor units, you would create too much force and move the arm instead of just adjusting for the extra weight.