I forget if you participated in the dry food / wet food thread WonderWoman - but have you tried wet food instead? It fills the cat up with liquid (which is more natural for them) and they therefore get less calories, so they gain less weight.
It's for example the difference between a human eating a plate of pasta, or eating a large salad which could be equally filling but have less calories.
Our cats definitely gain weight if we feed them dry food.
