Why would dog lose weight with a voracious appetite?

Honestly you need to visit your Vet to have your dog checked over. This can be a sign of a cancerous growth. The cancer "feeds" and it can take more and more food, and snacks in between their regular meals, to keep a dog up to its regular weight if it is fighting such a thing. We have gone through that with 2 of our labs over the last 4 years. It is tough to deal with.