I don't know if it has anything to do with being fixed, maybe its just his age, but its good you noticed and caught it early. Now you can watch him more closely and adjust the amount of food and exercise if you need to. I didn't really notice Sam was getting so heavy and now its really hard to get it back off him. I used to get so mad at people who had grossly overweight animals. I thought that they controlled the food so just fix it. But Sam has been on very little food for quite some time and gets NO table scraps and just a couple tiny homemade treats here and there and it just doesn't seem to be budging. He gets very little exercise though because he was sick and couldn't walk, then it got cold and my asthma acts up too much in the cold air. Jenny gets him moving a little more. He's still on prednisone also, so that doesn't help. Anyway watch him close so you don't have to go through trying to get the weight off.