Why are airline meal portions so small?

They aren't!

You are typically going to be sitting down and getting very limited exercise. On a long haul flight of 12 hours, based on a 2000 kcal/day requirement, the average person will burn in and around 1000 calories.

A conservative estimate for the calories in that small breakfast picture would be around 300, and that's a New York to London flight, which is in the 6-7 hour (i.e. 500-600 calorie) range, and on which another, more substantial, meal is also served not long after takeoff.

Add to that calories from drinks and snacks and I'd suspect that the typical passenger over-consumes on a long-haul flight.

I think most people choose between airlines based on the cost of airfare, and not the quantity or even quality of in-flight meal service, therefore airlines are not incentivized to compete on those.

Then their goal is to serve food that is just good enough for the majority of passengers. Increasing portions to satisfy the hungriest passenger on every flight would be inefficient, since at that extreme every other passenger would leave some part of the meal uneaten.

Frequent fliers who dislike the quality of airplane food just bring their own food. You could do the same, and bring your own supplementary food if you aren't satisfied with the quantity of airplane food. Why spend an extra X dollars on a more expensive plane ticket merely for more food, when you could just bring X dollars of (non-liquid) food onto the plane? The selection of food off the plane, and especially outside the airport, is certainly much better than the selection on the plane.

You could also ask the flight attendant for an extra meal. They usually have a few extra, and that is one way they can sufficiently serve the passengers with big appetites without wasting a lot of food on the rest.

In addition to the good answers do keep in mind your ticket class!

First and business class food is in quality good to outstanding, depending on the airline.
In terms of quantity, in higher classes you'll also be served multiple courses and / or have access to more at your own choice.

Additionally it is a logistics problem as well.
The more food is stored, the less paying cargo can the plane carry.

Since the OP has a strong opinion on this matter and has been seen debating that the suggestions given already are wrong, here is exactly what the OP doesnt want to hear.

Economies of Scale.

That’s the answer to this question. Google will help with that term. And also, an airplane is not a hotel where we primarily go to feed our children. If your children ‘go hungry’ on flights because the meal portions are small; You are definitely not well prepared.

Besides, airlines do allow you to chose your meal preferences well in advance specially for children and at that point you may be able put in a request for a larger meal.

I'm not technically a frequent flyer but I've flown quite a bit (more than 100 long haul flights, 8+ hours, say).

In my experience, people very rarely finish all their food. So my anecdotal experience contradicts the assumption in the question: the portions look fine related to what the average passenger consumes.

Technically they're not large or small. They are appropriately sized for the service, passenger demographic and city pair of the flight.

The airlines continuously work with their caterers to size meals and other provisions to feed passenger and reduce weight and...wait for it...food waste. Yes, for every person who 'asks' why meals are so small, there will be one who will demand airlines address food waste. Remember, everything not consumed during the flight has to be disposed of.

Anyway, if you think the single serving is too small, the easiest solution is wait for the flight attendants to complete the service, then ask for another one. Note, you can ask whey they serve you, but will likely ask you to wait until all other passengers are served first.

The second easiest solution is to book a higher class of service where plenty of food available, sometimes even a buffet.