How do animals stay so slim??

Axl is skinny. He's got more meat on his bones than he did when we got him, but he's still skinny. He eats like crazy so I wonder how he stays so slim. Maybe he's taking clinislim and I don't know. Also, dogs are programed to sleep about 16 hours a day, you would think that would make them have a tendency to gain weight, but no. I know dogs get overweight, at one point Rocco weighed over 100 lbs. Right after we had him neutered he just ballooned up. It was crazy. We worked though and got his weight down to about 80 lbs. He was still heavy but to get him any lower we would have had to starve him and I didn't want to do that. Okay, why am I talking about this.......