John ate way more at dinner than he should have.
Why do you think that's the case?
He is a glutton and never shares with other people. He always eats everything.
You should mind your own business when other people overeat. You can buy your own food and enjoy it without other people.
I can't believe he's eating that much, he should get rid of his fat and lose weight.
That is not your call to make.  If his overeating is preventing others from eating, you have a right to say something, but it is not your place to give John dietary advice.