I don't like wasting food either. But that's not a reason to over-eat.
I'd rather eat a smaller lunch followed by an afternoon snack (lunch leftovers?)
If I'm at a restauarant, I eat as much as I want, then take the rest home.
At the mall, I get the meal in a take-out box (styrofoam with lid) so I can take the leftovers with me.