Has the dog been tested for hypothyroidism? My Springer got fat, and no matter how often she was walked and how little we fed her she just kept getting fat... we thought it was because she was spayed, but that wasn't the case, when she was 6 she nearly died. The vet never suggested that we do blood tests, even though she went to see them every year... I think they thought she was just spoiled. We nearly lost her due to extremely high cholesterol, and her eye bulged out...
BUT I have had 6 other dogs, 3 neutered, 2 spayed and one intact, and though we have a hard time keeping Scout and Zoe trim, my 3 neutered boys have never had issues keeping trim.