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.