We have this problem, currently have 3 girls and one boy. The wake up call was when I found out he'd gained 30% of his body weight between 2 vet appointments a few months apart, when he was about 2 YO, so probably fully grown. I had noticed he was getting a bit fat.
Problem is that he eats his food, his sisters' leftovers (they generally eat about 2/3 pouch each) and hunts so probably eats a mouse or bird most days too.
So annoyingly, one solution has been to give the girls less food. They get two pouches between the 3 of them and then they eat everything with no leftovers. I know that Mumsnet abhors penis portions and I am suitably ashamed of this. They do have free access to dry food, which one of the girls actually prefers and fortunately, fat boy cat seems less bothered about.
You could try different food, some with less grain and more protein. Mine eat mostly Felix AGAIL or Sheba, which is not my preference but we also get Carny, Smilla and Bozita tetrapacks from Zooplus and I try to get them to eat some of this too.
These may seem more expensive, but they eat less of it, so probably works out about the same. We also get Smilla and Iams dry food and have had Harringtons in the past but they seem to have gone off that.
Zooplus are good for trial packs if you want to try different things, but don't fall into the trap that I did, and buy big packs because they work out cheaper per kilo, because we all know that with cats, the trigger for them going off a food that they've previously loved is the purchase of a large amount of it (I foster rescue cats, so have lots of opportunities to try and get a cat, any cat, to eat some of the backlog, currently have a nursing mother of 5 who's going through about a kilo of wet food a day and fortunately seems quite happy with the stocks of Carny and Smilla that my cats have decided are not to their taste).
You could also try the chipped cat bowls, that only allow them to eat their own food.