Social proof. Your busy colleague has already been vetted as "someone who gets things done" and therefore has been issued more work.

My answer to that would be that a busy person tends to work quicker (always on the move because of the pressure of so many things going on) and ends up being more efficient and driven by deadlines.

So they can squeeze in an additional amount of work. The person with very little to do should in theory be able to get the work done. But they have no deadline since they have so much time to complete. I guess this thought ties in somewhat with the saying "work expands to fill the time available for completion".

This is one reason why deadlines are important. Deadlines force people to take action in order to not miss a deadline. And become very motivating. At least that is what I find from personal experience.

When negotiating or making a deal I always use deadlines in order to get people to act on something right away. Otherwise (as once again the saying goes) "time kills all deals".

A productive AND busy person is organized, by definition. Productive OR busy may not get you anywhere.