It's possible 20lbs is closer to your cat's ideal weight, since some breeds tend to run large. Mine only hit 18+lbs while we freefed her. Now she's around 12lbs, in good body condition (just a little more abdominal fat she could stand to lose).

We really had to push the weight loss because it was clear she wasn't meant to be so big. Her asthma was significantly worse when she weighed more (multiple attacks a week vs now I don't even remember the last one within the past year), and there were huge patches of her body covered in greasy dandruff because she couldn't groom herself. It would have eventually led to a lot of problems. Better for us to put up with her antics than to watch a painful death, really.