Most Americans love to buy a lot of coffee—I learned this when I traveled through the country with an 8 oz reusable cup. It takes about 10 to 15 minutes to drink this amount of coffee, by the time you reach the last drop, it's almost cold but not quite. I think that's a perfect size. But, the cafe staff I met did not.
In the San Diego airport, I asked for "this cup, full of coffee." The staff examined my cup like a foreign artifact. He wondered aloud how big it was. When I told him 8 oz, he looked confused, and then charged me for the smallest size they have—12 oz. I paid and didn't say anything.
Then in the Los Angeles airport, I asked for the same thing. The young woman working there was equally surprised by it. She looked at the till, frowned, and then looked over her shoulder, and said, "They're just gonna overcharge you." So she took the cup and filled it with coffee for free. I guess, to her, that's fair. I said thanks.