I have never seen that at a party. But, I have seen where people invite a couple of guests over for dinner, but do not put out any more food than for just the two of them. Actually, I have only seen that with older people. I don't know why. I don't eat an abnormal amount. 1/2 to 2/3 of a restaurant meal is usually perfect.
Due to slowing metabolisms, older people may have to cut back the amount of food that they eat to avoid gaining (substantial) amounts of weight. Or their appetite may have naturally declined since their taste buds and sense of smell have declined (especially bad if they've smoked tobacco).
So - what they "see" when they look at an amount of food is how many servings they would get out of it. Without realizing/remembering that the teenaged boys in the family could wolf the entire "four to six servings" down in one meal and ask for dessert........
Or that someone with diabetes, low blood sugar, or other medical conditions might find the "servings" acceptable but need to eat AGAIN in three hours - five or six times a day - not just three meals. (I represent that remark.)
It depends on the age, gender, appetites, activity levels, and medical conditions of the "guest".
I would swear that having food left over is easier in the long run than running out of food - as microwaves are in many homes and simplify fixing a snack or small meal later if someone else shows up hungry. But I grew up with grandmothers who went by the "is the table and part of the kitchen counter completely covered except for barely enough space to sit and eat?" method of determining "enough".
We had to eat at least half and maybe two-thirds of the food for it to fit back into the fridge after the meal was over. More cans or frozen food would be prepared to eke out the meager remains for the next meal..............(ha, ha, ha). Or the leftover meats would be sliced and bread, cheese, tomato slices, onion slices, pickles, and lettuce would be left out with condiments for a "light meal" of sandwiches...........usually sandwiches which Blondie's Dagwood would have envied in their towering approach to battling hunger.