Good explanation over on the PetMeds site of your cats Primordial Pouch i.e. the saggy belly that so many cats have, even those that are not fat i.e. our Fleur (sorry Oscar and Dusky can’t include you in that comment!).
Basically this lump of fat and skin is a perfectly naturally part of a cats anatomy and has nothing to do with whether a cat has been sterilised.
Some cats have it (Fleur, Dusky, Oscar) others don’t (Moreno, Saidi, Kasper and Sophie, and Saidi gave birth to several litters as a young cat).
This bit of loose skin and padding at the belly provides extra protection and insulation to your cat during fights when a cat’s practice of “bunny kicking” with the rear paws could result in severe abdominal injury to their opponent. In fact, cats have excess skin covering the entire body which helps them squirm out of the grasp of other predators.
Another function of the abdominal flap is to allow the cat freedom of movement to fully stretch and extend the back legs when in full stride.