Neutering can decrease roaming behavior, marking behavior, and some forms of aggression, but genetics and training are more important contributors to your pet's behavior. People think neutered dogs are more sleepy and less playful than usual after castration. In fact, those behavioral changes are typical of puberty, the stage of life when most neutering occurs. 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.