I have two cats; a Siamese, aged 5, he weighs approx. 5.5 kgs, maybe slightly more now, and a DSH aged 2, he weighs 4.2 kg. I feed them high quality wet food, brand Bozita, and Vets Kitchen dry food.
I have been feeding them wet food when they want and dry food down all the time, but YC remains at ideal weight, Siamese gets fatter!
I've removed the dry food for now and am just putting a little of it down for the night.
How much do you feed your cats when they are on a diet?
The vet recommended Royal Canin dry food Lite, but I don't want him to eat too much dry food.
At the moment I am trying to limit how much I feed him, but he is constantly asking for food. To punish me for not feeding him, he is constantly fetching my knitting out of the bag and chewing it! 
He does go out, but not much in this weather. The only exercise he gets is chasing YC up and down the stairs
I play with him a little, but as soon as YC joins in (which YC does almost immediately) Siamese won't play anymore 
I don't want him to be hungry or miserable or to think I don't love him anymore, but he does need to lose weight.