Neutering makes pets gain weight and become obese.
Within several months of neutering a dog or cat, metabolic rate begins to decrease, and thus the maintenance energy requirement(MER) also decreases. However, most owners continue to feed the pet the same food, using the same amount and in the same manner as before neutering. Hence, shortly after the surgery, the pet is consuming more calories than are required to maintain an optimal body weight.
Yes, there is a relationship between neutering and obesity, but if feeding instructions after the surgery are modified to reduce caloric intake, obesity can be avoided.
If you have more specific questions - Ask the Nutritionist.