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.

But why is it wasteful if you intend to eat a portion of your dinner for tomorrow’s lunch? All the food is eventually eaten.

Is it because a small paper box is involved? Would you find it less obscene if everyone carried a reusable food container with them to a restaurant to mitigate the risk of offensive boxed leftovers?

This whole discussion feels like "why would you need debuggers, you should not introduce bugs anyway". Even if you are careful you will eventually end up in a situation with leftovers; it may be your fault, but it may also be the restaurant’s fault.

Not really, because different people are different. Some may not have the capacity to eat fewer large portions - like me - and eat less, more frequently.

Many people eat more than they need to at any given time, that is arguably greedy and argubably wasteful in a different way.

Portion sizes are static, appetites vary, letting people manage for themselves is perfectly fine.