Are their tummies distended? It's hard to tell in that position but it does look like it.
My first suspicion would be worms, which can do that, if they are having no problems peeing or pooing. Go to the vet, get a stool sample done, and get PRESCRIPTION (not overthecounter) meds to treat them. You should treat them all, since if one kitten has it they probably all do, especially since you've said you have two that look like that. A bad case of worms causes a big stomach, a moderate or mild cases causes no outward symptoms.