If you've discussed spay or neuter options with your vet, they may have mentioned that spay or neutering your dog can cause an increased risk in obesity. This is caused by the loss of estrogens and androgens (sex hormones) from the spay or neuter surgery that causes a decrease in metabolic rate. The decrease in metabolic rate has the direct correlation to pet obesity. In addition, estrogen in known to decrease appetite, so the loss of estrogen can cause some pets post surgery to be hungrier (more information here).
There have been studies done that prove that if dogs that are intact (not spayed or neutered) and spayed or neutered are fed the same amount, the dogs that are not spayed or neutered will gain more weight (more information here).
The Whistle Food Portion Calculator takes your pet's spay or neuter status into account when providing a food portion suggestion to recommend a portion that will help your dog maintain an ideal weight.