Fixed cat is gaining lots of weight - Why?

It changes their hormones and metabolism such that they tend to overeat (hormone that tells them to stop is not as strong) and at the same time, need less food (slower metabolism). If it were me, I would introduce the cats to meal time. Put food out two or three times a day, let them eat their serving, then put all the food away. It's an adjustment but most cats figure it out within a few days - eat when the food is offered. Then you can easily control how much is allowed to eat.