The big number decrements for every increment/decrement away from the initial numbers on small stones. So on the small stone if you if you either add or remove 2, the big stone reduces by 2, but if you undo your change, ie. you add one to a small stone, then remove it, the big counter goes -1 then +1 back to original state.
It's not super intuitive, I should have reserved more time to explain what's happening. Thank you for playing though, much appreciated!