One way to test this would be to use a scoop that's 2x bigger than the other scoop, and putting in 3 scoops using the smaller scoop vs. 2 scoops of the bigger scoop.
Tried that by using three half-ish full scoops against two full scoops. He always goes to the bowl which has had the highest number of scoops in it, even though it might actually have a lower quantity of food in it. He seems to be counting rather than comparing quantity, because his trigger is the other bowl getting +1 scoop versus his.
Can we measure when he loses count by using many small scoops? I expect if his current bowl has passed a certain threshold of scoops he won't bother switching no matter how many scoops are going there.