Weight gain but not overfeeding

One of my cats, who is nearly nine years old, has been gaining weight/size.  I do not overfeed him but he is getting bigger -- last time I took him to the vet he had gained half a pound but the vet was not concerned.  However, he seems even bigger since then (a few months ago).  Has anyone experienced this?  I don't feel any masses/growths on his body but am wondering if there is an internal problem.  He is not constipated and is urinating normally (he eats a urinary tract formula diet to prevent crystals from blocking his urethra).

Thanks!

Did they check his thyroid? It's very likely he's just slowing down. Have you tried cutting back his food, just a little?