The saggy skin in front of a cat’s legs is an evolutionary feature, not a sign of an out of shape cat.
The primordial pouch is the formal name for the saggy belly cats have. Some breeds have a more prominent pouch than others, but from big cats to house cats they all have it. It’s a loose flap of skin & fat which is the result of evolution and not the because of spaying or neutering (again, big cats such as lions and tigers have it too and they aren’t spayed or neutered).
One use for this extra belly skin is abdominal protection from the rear paw kicks of other cats. Another benefit is it gives cats greater flexibility (especially when they sprint or jump). It is also theorized that when early cats would find a meal this skin would stretch to allow the belly to expand and hold more food.