The empty pouch of skin hanging from a cat's belly is normal. If you look at pictures of cheetahs and lions, you will notice they have a pouch too. All cats do, but on some, it's more obvious. This is extra skin to allow the body to stretch wide open when they are in a full body run. The front and back legs stretch way out curving the back as it runs. That pouch provides extra skin so the cat can do this comfortably.
It could be as a result of the cat being overweight as a result of over feeding, also when cats age they tend to have loose hanging bellies. Neutering also contributes to a flabby stomach however you should take the cat to the veterinarian to rule out an underlying disease.