Studies have shown spaying and/or neutering before growth plates have closed causes many health issues. It is recommend that you wait till they are 18 to 24 months old and never before a year of age. The link at the bottom of the page explains one study.
Obesity is such a common problem of dogs and cats that many veterinary organizations and pet-food companies provide multiple resources and special diets to help clients reduce the weight of their pets.
Obesity is influenced by a number of factors, and while veterinarians report that neutered animals do have a tendency to weigh more than intact animals, it can occur regardless of the animal’s age when the surgery was performed. (A published study in 1991 indicated that dogs did not develop obesity when they were neutered pre or post puberty. Another study from 1996 showed that cats can gain weight after pre or post pubertal gonadectomy. The 2004 Cornell study found a decrease in obesity for both male and female dogs who had undergone pediatric neutering compared to those neutered after 5 months of age.)
So obesity is a multi-factorial problem and not an automatic consequence of neutering – even an intact animal can become obese if a proper diet and exercise regimen is not followed. Just as in humans, dietary indiscretions and lack of activity are the real culprits in this case.