Of course there is a lot of fur around. Can you see Willy's sides a little sunk in when you look from above when he is walking. That would be a good sign.

Our Tigris is a cat with a big framework. He is 13 lb (5,8 kg) all his life and has a belly. Every time the vet looks at him she is not sure whether he's overweight but then she agrees he's not. So he's borderline fluffy But you still can see his sides sunk in when he walks.

Is Willy big from the bones? Like his Maine Coon granddad (or whoever it was )? If he is small then 10 lb may be too much for 9 months. He should gain his full weight at about 15 months.

But I agree on the gorgeous chunky trio- looks as if they are long lost triplets