Researchers from Bristol University think they know why. And it's all down to our perception of foods, including which ones we think are the most filling, as well as our perception of how much we believe we're eating. Putting it another way, it could be possible to fool ourselves we're eating more - when actually we're eating less.
In tests volunteers said they felt fuller for longer when they thought they were getting bigger portions. In one experiment they were shown the contents of a fruit smoothie they were going to have a few hours later. Group A saw a small portion of fruit, while the researchers showed group B a large portion. Both groups received exactly the same smoothies with the same amount of fruit - yet those shown the larger portion said they were much fuller afterwards than the small portion group.
So is it a case of mind over matter when it comes to appetite? Do our beliefs and expectations play as big a part in what we eat as the size of our portions and how many calories they contain?
If so, how could this information help us eat more healthily? By labelling food as 'satisfying' or 'hunger relieving', the researchers say, and by not using terms such as 'lite' or 'diet', which might fool us into thinking we're eating less.
Do you agree? Does what and how much you eat have more to do with your brain than your stomach? Or is it the other way around? Tell us what you think.