My Ben was one who put on weight after neutering. I had always free fed and his appetite was obviously greater after surgery. But I had also gotten a puppy and Ben keeps getting into the puppy food (which is higher in calorie). Just having the new puppy seems to make him want to eat more -- it is more a contest than a hunger. So -- I am not sure if I can really blame the neutering alone for his weight gain. I have not noticed a slowing of his metabolism as far as activity either. Still ready and willing to play, chase, and be chased. He was older when I neutered him, 4 years old. I think those neutered before a year or two old have a greater chance of NOT gaining weight.
He gained a 1/2 pound in the first 2 weeks after surgery -- which is quite a lot in a short time. He continued to gain a bit more, but then I got on top of it. I am giving very low calorie treats (carrot, green beans, and a weight management treat). I changed foods and am making a more concentrated effort to exercise him outside. Keeps me warm while I wait for them all to potty so a nice side benefit too.
He is losing the excess slow but sure. Others do not think him overweight, but I know how lean he was before and that is what the vet said was healthiest for him. Not all dogs gain any weight at all after neutering and I think if they do put on weight, it can easily be a temporary thing. Since you are paying attention ahead of time, you may not see the affect at all. I tended to pamper him with additional treats both as a means of keeping him happy after the puppy addition and to make him feel better after surgery. Food should not be used like that for dogs anymore than it should for people. Emotional eating usually results in unwanted weight. So, I am saying -- I am more to blame than the surgery. I am very thankful for the surgery. He will never be turned into that crazed little horndog when faced with a dog in heat (and too small Annie is safe from unwanted pregnancy), no more marking (worked for him but doesn't for all), and he will never worry about testicular cancer and has a very reduced chance of prostrate cancer. I think he is more loving than before. I always kid about my selfish Ben -- he is with the other dogs -- but with me he is the sweetest dog possible! I would do the same if I had to make the decision today.