Please take your cat to the vet. DO NOT WAIT. My female cat tripled in size in just over 1 month. She wasn't eating but still got bigger. We were stupid in thinking we could wait until after the holidays to take her to the vet. She died in her sleep early this morning. I have to live with this for the rest of my life. She was a beautiful, sweet cat and didn't deserve this.
Hi, I have had cats my whole life and I haven't seen that rapid weight gain in any of my cats. There are a few possibilites here. First, as PK said, hypothyroid disease could be a cause, liver disease-causes swelling in the stomach, and any disease that could cause swelling/fluid build-up could cause weight gain! Or, it could be innocent and the cat's just being a pig!! =D
If the cat's being a hog, then you're doing the right thing by giving him lower calorie food. From now on, I would start feeding him wet food everyday. But, to be honest, that rapid weight gain makes me nervous. Just to be safe, I'd go to the vet. Watch your kitty like a hawk to see if there are other symptoms.
Please keep us posted!
You said he had food available all the time? That might be your issue right there. Cats should eat twice a day, or once. Feed him the amounts that he'll eat in 30 minutes. After 30-45 minutes, take away his food and feed him again at night. Your cat's diet and eating schedule seems to not be controlled. Hard food tends to cause obesity in cats, and Iams or Sciene Diet, or Purina will certainly not help him lose weight. In addition to that, you should get the cat evaluated by a vet. Some cats have hypothyroidism, which is rare in cats, but it does cause rapid weight gain.
I still think, though, that adjusting his schedule should do the trick for ya. See if you can incorporate canned foods with higher nutrient quality. Iams is not species appropriate. We have a document written by our community leader, Savas, which talks about canned foods and how important it is for our felines to have moisture in their foods. Go with brands that are more suitable for felines, with human intended meat. Cats function with fat, not carbs, which is primarily what you're feeding him. Carbs make people overweight if consumed at high levels. It is no different in cats.