Even for zero calorie things like water and unsweetened teas/coffees. You just get smaller cups. I'm not even sure you can get a US large in Japan for most drinks?
When I regularly visited New York for work, and we'd get takeaway sandwiches, I'd have to open them and remove half the filling. I just couldn't physically eat that volume of meat, cheese or especially mayonnaise. For all drinks, I'd order small.
Maybe I got too used to some of the obscure burrito places around Atlanta that would put way too much on them?
People generally split a Pastrami sandwich over a couple meals or with someone else.
Now, there are plenty of food vendors and restaurants in the US where big portions are considered part of the experience. Especially hamburgers, subs, and other sandwiches. I once ate at a place that served a plate-sized burrito completely covered in french fries. 12 inches wide and 6 inches tall. SOME people can eat that amount of food but most people cannot, and nobody is expected to.
Finally, large portions in NY street food are often customary because for lots of people with demanding jobs and 12-16 hour shifts, lunch is often their only meal. Or, half of it is lunch, the other half is dinner later on.
I don't know about "the US", but as a "European" I thought serving sizes were comparable to what I get in restaurants at home. Drinks were an exception, since basically all restaurants had unlimited soda for next to nothing. This was actually great, since I was riding a motorbike in the desert in July.
For reference, I live in France and visited LA and random towns in the western states.
And on the drinks, even places in Japan that had free refills still gave, at largest, an 8oz cup. Usually, I think they were even smaller. Even getting popcorn at Universal, the bags were large, but nothing compared to what I'd expect over here.
Some of this, I'm sure, is having gotten used to ordering the larges. For a time, it was not unheard of to get a 32oz soda at any given convenience store. May still be normal? I don't know.
(And, of course, this isn't getting in to the sizes of vehicles.)
But then, I think it's only been Americans I heard this from, so.
The coffee was the one that really surprised me. Order a coffee and get a 6-8oz cup. With nothing on the menu to indicate you can get a 12-16oz. Was surprising. (Not bad, mind. Just surprising.)
I've seen what large US drinks look like, and you definitely can't get that here in New Zealand. Like a litre of soft drink at a fast food place, it's absurd.