The changes merely reflect your pet growing up. But don't blame obesity on being neutered. Any dog can become obese. Obesity should be controlled through proper diet and exercise. The list of medical reasons for castration is convincing.
Testicular tumors are very common in older dogs.
Unfortunately, castration does not prevent prostatic tumors. Owners should castrate dogs who carry heritable diseases, such as hip dysplasia or other congenital conditions, to make sure that undesirable traits and disease aren't passed on. Cryptorchid dogs are at much higher risk of developing testicular cancer, and the condition is hereditary. People of both genders understand the benefits of spaying female dogs, but there still are a few myths out there. It is not true that it is best to let your female have a litter of pups before spaying her.