If your dog gains weight and you know he's eating a reasonable amount, make sure to test him/her for Hypothyroid. It's very common in older dogs. I had two dogs with Hypothyroid. The first time it happened, I just thought I had an old fat dog, but when he was tested, he had it. Not that this is what the 77 lb dachshund has. Might just be overfed. But something to look for.
Al and Kim
Village of Fernandina