Two meals containing similar amounts of energy can produce very different levels of hunger two hours later. Fullness is generated by several mechanisms, and energy content is not the main one.
Volume and stretch send the first signal
The stomach carries receptors that respond to distension, and that stretch information is among the earliest signals reaching the brain during a meal.
Food with high water content occupies more space for the same energy, which is why a large bowl of soup registers differently from a small dense snack of equivalent calories.
This signal fades as the stomach empties, so it determines fullness during and just after a meal rather than several hours on.
Protein produces a disproportionate effect
Across a wide range of comparisons, protein-containing meals suppress subsequent appetite more than meals of similar energy dominated by fat or refined starch.
Part of the explanation is hormonal signalling from the gut, and part is that protein digestion is comparatively slow and energetically costly.
The practical consequence is that the same energy intake distributed differently across a day can leave someone noticeably more or less hungry, without any change in the total.
Fibre changes the timing of digestion
Fibre adds bulk without adding much digestible energy and slows the rate at which the stomach empties into the intestine.
That slowing spreads absorption over a longer period, which flattens the rise and subsequent fall in blood glucose that often precedes a return of appetite.
It also feeds bacteria in the large intestine, whose products appear to participate in appetite signalling, though this part is still being worked out.
How fast a food is eaten matters independently
Fullness signals take time to arrive, so a meal eaten quickly can deliver a large amount of energy before any of them register.
Foods requiring chewing are eaten more slowly by necessity, while soft or liquid foods can be consumed faster than the signalling system can respond.
This is one reason that drinks contribute energy with comparatively little effect on subsequent appetite, and why the same food eaten at different speeds can feel different.
Expectation contributes as well
What somebody believes about a food influences reported fullness, and labelling the same portion differently changes how satisfying people report it to be.
This does not override the physiological signals, but it sits alongside them and helps explain why identical meals are experienced differently in different settings.
Anyone whose appetite has changed markedly without an obvious reason should raise it with a doctor rather than treat it as a matter of food choice.