Restaurants are so wasteful. Huge amounts of food is thrown away both in the kitchen and in the form of uneaten plates. Just going from restaurants to home probably saves a decent amount of food.
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.
That said, I'm surprised taking leftovers home isn't an option. Around me, at least, the expectation is that you do take leftovers home.
American portion sizes are ludicrous but most restaurants won't ever bat an eye at wrapping up the rest of your food for you.
Even a $200 per head restaurant will do it. The only places that won't are the high end places where you're getting a $400+, 19 course tasting menu (+$150 or more per person for wine).
I do it every outing here in the US, though! Our portion sizes are ridiculous.
Food waste in home vs. restaurants is more difficult to compare. Many people will eat food that restaurants would not consider serving, simply because they have no other choice. On the flip side, there are very wasteful household that buy far more than they will consume.