23 lbs is a large cat but it all boils down to what built he is. If he is fat you can switch him over to a low fat food or ration his food.
I put my little Smokey on a diet (ration his food) and he started to lick himself bald. I then switched him to a low fat food and is doing better. He only weighs 14 lbs but is a small cat. I would like to get him down to 11 lbs. so he isn't as round as tall. He now has lost 2 lbs.
My cat Henry weighs 18 lbs and is not fat at all. This cat is a big one!
I will post a picture and you can see. Smokey is the fat little black and white one and Henry is the big white one.
My 5yr old cat is 17lbs she is starting to get a little bit chubby but for the most part shes all muscle. So I guess I would have to know if your cat is fat or muscles if shes fat well I have a friend of mine who had to put her cats on a diet because they were 35lbs. their now down to 25lbs and she says their still chubby.Hehe I love cats.