My doggies

It's almost 2 weeks now that I have been walking my dogs (actually it's just Rocky) because he is just gaining weight. I already put him in a diet and he is still bigger than he's supposed to be. Before I started walking him on the empty lot next to our store, he was 20 pounds and now he's 18.5 pound but at his kind (Papillion) at the most should be 12 pounds. We are a little bit worried because he started to pant more than he used to. They pant a lot but we think Rocky pant too much because he is getting fat. Well, aside from my regular gym session, I have to walk 10 rounds on the field (at least it's not that big of a field).