It is all about how many calories a dog, horse or human burns. We have a 22 pound jagdterrier who gets 2 cups in the morning, 2 cups in the evening and if he is hungry he will eat a lunch with my husband at the shop. But he goes 90 miles an hour during his waking hours. He hunts, follows the tractor, 4 wheeler, patrols the farm, etc.... He just burns what he gets. He is actually hard and very fit too.
Congrats on taking the challenge to what is right for your heeler. I often recommend people put their dog on a diet but it isn't that often that people take heed to what I say and actually do it. Good for you and good for your dog.