Are you serious?? I'll ask the same question everyone asks me when they see my 20lb cat... what do you feed that cat? Is he/she diabetic by chance? My cat, Henry, is diabetic and gets two shots of insulin a day. I don't feed him very much at all (low fat diabetic food) but he still keeps the weight on. Here he is...
One night, about 5 years ago our fat cat got in to a bag of Hershey's Kisses left on the end table by my now Father-In-Law. He not only finished off all the chocolate in the bag, he also downed the wrappers. This caused a major urinary block, and he is now on reduced magnesium food for the rest of his life. This is where he started to blimp up. He eats about 3/4 - 1 cup of food a day. He's not active, just sleeps on the bed on all day. He doesn't have diabetes, but I'm sure it's something I should be looking for. How was your cat diagnosed?