I have had 2 of my girls spayed when they were 6 months old and the last one will be done next month when she is 6 months old. None of them have gone into heat. The 2 that have been spayed did NOT gain weight.
The weight gain comes more from people over feeding their dogs when they reach the age when they are not growing any more and don't need as much food, which is around 8 months to 1 year old.
Gaining weight after a spay is a myth, it's more the owners fault than the dogs.
Sandy, Koke and Lelaska's Mom