It's not that cats don't, sometimes, get fat after being altered.
It's that cats getting fat don't have much to do with them being altered.
We generally "fix" the cat at a young age, when their bodies are still growing, and they eat a lot to fuel that important growth. But just about the time they start maturing, they no longer need that much food. If they are eating too much, they will get fat.
And people blame it on the altering.
They reach this wrong conclusion because it's much more common to see a fat, altered, housecat than it is to see a chubby stray who is still reproductively active. But that's not their hormones. That's their circumstances.
That lean stray probably isn't eating enough. It's one thing for a cat to live in the wild and hunt small rodents, as they are designed to do. It's quite another for a cat to try to make it on our city streets.
The real reason housecats can become fat is that they are being fed the wrong food. This is even more important than exercise, which cats should also have. But most commercial cat foods have grains as the first ingredient on the label.
Go ahead, check the label.
Why is this so bad? Cats are hunters. They don't eat grains. When fed grains (sprayed with a meat sauce to make them think it's food) they get fat. Because they don't have the enzymes to digest grains.
All it does for them... is make them fat.
So we certainly shouldn't blame altering for making our cat fat. It was us.
We can stop the cat constantly asking for food by feeding them properly. Read more about how to slim down a fat cat.