Hello Keith, I'm Dr. Deb.
I recently came online and see that your question about Tiger hasn't been answered. I'm so sorry that you've had to wait for a response, but if you still need assistance, I'd like to help if I can.
Certainly one possible explanation for Tiger's enlarged abdomen would be roundworms which is the most commonly seen internal parasite in kittens and puppies. It's fairly easy to treat this parasite with an over the counter wormer which can be purchased at most pet or grain stores.
The active ingredient is called Pyrantal Paomate and is sold under different names such as Strongid, Nemix or Evict. I recommend treating all of the outside cats, if possible, two weeks apart for three doses.
Since your inside cats don't have exposure to the outside ones, it wouldn't be necessary to treat them in most cases although it is possible that eggs could be transported inside your house on your shoes. Therefore, it might be prudent to worm the inside cats as well. Even if they don't have parasites, Pyrantal Paomate is very safe and wouldn't cause any harm.
I should mention another possible explanation for a distended abdomen in a young feral kitten which would be Feline Infectious Peritonitis (FIP). This condition is typically seen in kittens or younger cats who start to lose weight, often stop eating and look debilitated (in addition to the distended abdomen). Your inside cats would not be at risk for developing this condition if they have no contact with the outside cats.
I hope this helps and that Tiger only has parasites which can easily be treated; again, my apologies for the delayed reply. Deb