“The portion sizes are so much bigger in the U.S.!”
You are eating lunch at a restaurant in the U.S. with a friend, and the restaurant brought you a huge plate of food. You are amazed at how much food American restaurants serve, so you say this.
The portion sizes are so much bigger in the U.S.!
A "portion" of food is the amount that one person is supposed to eat for a meal. If you want to talk about the amount of food that a restaurant, cafeteria, etc. gives you, you use the phrase "portion size". "Portion size" is the amount of food that is given to each person for a meal.
If the weather is really hot, you say that it's "so hot". "So ___" is a way to express an adjective very strongly.
When you want to compare two things, you say "___er" or "more ___". So, for example, you'd say "It's hotter in New York than in London."
Putting these together, if you want to compare two things, and say that the difference between them is very strong, you say "so much ___er":
It's so much hotter in New York than in London.
When it's clear what you're comparing to, you don't need to say "than ___". So, if you're from London and visiting New York, you could say:
It's so much hotter in New York.
When you're talking or writing about the United States, the most common and natural way to refer to the country is to say "the U.S.". This is much more common than calling it "U.S.A." or "the United States".
Notice that you always have "the" in front of "U.S.".