I think it is fairly common for both female and male neuter cats to get a bit of a loose hanging belly but may be not all of them. They shouldn't be fat and round any more than any other cat though, so if you're not sure a (not very flattering) clear picture of your kitties standing up and from the side might be a good way to get a clear answer. Both my tabby girls had wobbly bellies after they were neutered but played lots and were nice and fit until well in to pensioner years. The vet was always happy with their weight I'm afraid my 2 1/2 year old boy has one too but I think he needs a bit of a work out in a kitty gym, he's been getting a bit lazier of late!
Both of the (normal weight) spayed girl cats I've had/had developed little bellies - mostly fluffy and loose skin. I call it their "poof" and think it's pretty darn cute.