Would you like a box with that?

A noticeable feature of dining in American restaurants (well maybe not the posh places that I can’t afford) is that they offer you a box to take your left-overs away in. You can often get a second meal out of this (tonight for example) unless, of course, you forget to retrieve it and your car smells of curry for a month. No chance of that happening in Ireland or the UK. I think I saw some unfortunate American ask once in Dublin only to be sniffily refused.
So is it a “good thing”? Well the option of a box means that you get what you pay for or at least the gap between the value of what you pay and the value of what you consume is smaller. The cost of the box is probably small so an efficiency gain, for sure.
Or is it? I have this niggling feeling that there may be a downside. Without the box, you have an incentive not to over-order. If we assumed that people don’t know how much they want to eat or how much is good for them (otherwise why is there an obesity epidemic) then its less clear to me that having this option is welfare improving.