Three words do most of the work in this subject and they do not mean the same thing. A helping is what somebody puts on a plate. A serving is a conventional unit chosen for a table or a label. A portion is whatever a particular person eats on a particular occasion.
Confusing them is easy, because in ordinary speech all three come out as the same sentence. Most of the disagreement about portion size is really a disagreement about which of the three words is being used.
The serving as an editorial decision
A serving is a decision, made by whoever is producing a table or a package, about a convenient quantity to describe. It is chosen so that figures can be compared and so the arithmetic stays manageable. It is not a recommendation and it was never intended as one.
This is why servings differ between countries, between decades and between categories of food. They are conventions, and conventions get revised. Reading a serving as an instruction produces a confusion that the people who wrote the convention would not recognise.
Helpings and the shape of the dish
The helping is a much older idea and it belongs to the table rather than to the page. It has always depended on the crockery, the occasion, the company and the cook. Household writing from earlier eras discusses helpings in exactly those terms, with no numbers at all.
That older vocabulary is more honest about what is happening. A helping is a social act. It responds to who is at the table and what the meal is for, and no table of conventions has ever managed to describe it fully.
Why the words keep colliding
When a printed serving is compared with a real helping, the comparison usually goes wrong in one direction or the other, and the mismatch is reported as though somebody had made an error. Nobody has. Two different kinds of quantity are being set side by side.
Keeping the three words separate does not settle any argument about eating. It does make it possible to say which argument is actually being had, which is usually the harder part.