It's very much not in a restaurant's interest to waste food in the kitchen. Ingredients are a significant part of their costs, so restaurants worth their salt (hah) are usually pretty good about this. And this is starting to be regulated in some places as well.

Portion sizes on the other hand are entirely ridiculous. I often eat out with a tiny friend (in height and width). They tend to ask for smaller portions, but it's insane that many restaurants simply refuse to be amenable to that request. This means either I get fat eating their leftovers, or half a portion goes in the rubbish. Taking the leftovers home is rarely an option over here. I get that you probably can't serve half a chicken breast. But there seems to be no logical reason not to serve fewer fries, potatoes, pasta, whatever when a paying customer asks for it.