It's called a "fat pad" or a "fat sack" and it's purrrfectly normal. There are some theories that spayed females get it, but that hasn't really been proved out, as some males get it also. I think it's just one of those genetic body-type things.
Actually almost all felines have the "primordial pouch" - part of the resason for it is to protect organs in fights (the fat sack" is between their skin and vitals) this protects them, in the wild, from the wild "kicking" that often occurs in fights, in nature.
Another reason, is to allow FULL EXTENSION when leaping - if their tummies are TIGHT - they would not be able to get the same level of extension of their back legs, when leaping.
It is common for the "primordial pouch" to collect more fat, when the animal is neutered, because their metabolism slows down. BUT --- even SKINNY Cats have the pouch -- it's just not as obvious