Interesting! I’ve wondered about the saggy tummy on my boy cat – he’s had them ever since I got him and his sister, and they were fixed at about eight weeks if I rightly recall. The sister doesn’t have them. The impression I got from the vet was that his size was… impressive, but normal for someone given his genes. (good dose of Maine Coon in them)
Very few problems at all have been reported-- none that I know of regarding muscle development. Tempermentally, dogs show marked* improvement* if they’re fixed before sexual maturity, especially in males.
As for obesity, all elderly dogs are at risk because they’re less active. It’s easily preventable by monitoring the dog’s food intake and giving low-fat treats like carrots instead of calorie-laden biscuits. My dogs have all been free-fed since puppyhood and none of them have ever gotten fat.
The most prevelent issue seems to be that older spayed females can have slight risks of incontinence issues when they’re elderly. My ten-year-old female (the one who was spayed shortly before her first birthday) started “leaking” a year or so ago. When she would get up from a nap, there might be a small patch of urine. The vet put her on DES which is a canine hormone. Some dogs may have to take it for the rest of their lives, but mine only needed it for a short time before the problem went away. None of my other dogs have ever had problems with this.