A fully grown house cat is between 8 and 20 pounds, unless your cat is bone structurally very large, 20 points is already overweight. But sometimes it's hormones or lethargy that can cause a cat to gain weight. I have our cats on scheduled eating times and they don't get more food after their two meals aside from occasional treats. I have 3 cats that are the right weight and one that's overweight. He's overweight because he's a fixed boy and in their older age they gain weight because of lower testosterone. He is, however, only 14 pounds. That being said, there are circumstances where being over 20 pounds is not always the owner's fault like in the case of a thyroid problem