There can be many reasons that your dog is panting and gaining weight. As dogs age, they often put on a little extra weight due to a decreased activity level. The extra weight can make breathing more laborious which results in panting. Canine Cushing's disease is an endocrine disorder that can cause increased water drinking, frequent urination, a pot belly dog, thinning skin and hair loss. While some of the symptoms may seem like they are related to aging, these slow and gradual changes may also be a sign of Cushing’s syndrome in dogs.