Is this a thing where you are from?
I suggest here in Blighty, the waiters would be rather confused by such a request; not to mention something of a social faux pas.
In fact, it's not uncommon for the waiter to offer a box unprompted if you have a fair amount of leftover food.
Ah, this makes sense.
I recall eating in US restaurants and being given ridiculously large portions... something we half-joked - half-seriously suggested was down to the size of some of the patrons. Presumably is it instead the expectation that some of the food is taken home.
I'd imagine that it might differ based on where you are. Remember that the US is pretty close in size to the entirety of Western Europe. There are a lot of different places here.