The wasteful step is to order more food than you will eat. Someone not finishing their meal is what I don't recognise. If someone has a small appetite I am used to them enquiring about portion size and arranging to split dishes with others rather than expect to bag up an excess. There are always people keen to get their hands on anything going spare anyway.

I only see it happen if someone falls ill, is called away or there was an error in the order e.g. you manage to order four entire chickens instead of four portions.