This "feeling more full" idea is only relevant if people eat more calories if they don't feel full. If you eat a pre packaged processed meal and regardless of feeling full or not just stop there... then that aspect doesn't matter.
If we talk about calories in... the feeling full aspect only matters if you increase the calories in.
In other words, if we treat humans as a machine, in which you can simply flick a switch marked "stop eating when you have actually eaten what you need," everything's hunky-dory.
Unfortunately, humans are very much unlike that sort of machine. Even a human with a great deal of what we generally term willpower is, by and large, dependent upon his body's signals to indicate when he has eaten enough. When those signals are interfered with, it's not just a matter of being a good enough person to stop eating, it's a matter of how do you actually know when to stop?
As to a calorie not just being a calorie... if you were interested in surviving... as in not starving to death... a calorie would actually be a calorie.
Even then it gets a little dodgy, but sure, we'll grant this for the sake of argument.
But that's not what we're talking about, are we? We're talking about being healthy. In particular, we're talking about what causes people to gain or lose weight. And while yes, eating twelve thousand calories of pizza and not exercising will cause you to gain weight, and eating a small bowl of brussels sprouts and running a marathon will cause you to lose weight (and probably die ;-) ), there's a lot of middle ground where the details of the food you're eating (percentages of carbs, fat, proteins, etc, and what kinds of each of those) matter more for what you can get out of the food than the straight-up calorie count.
Do different calories get processed differently? Yes. High density food... food that has a lot of energy in it per unit volume tends to not be processed well by the body if you're sedentary. If however you are active, then you can eat high density food and not feel like a slug afterwards.
Try it. Spend a day being active all day... swimming or something. And then go home for a big meal. Then try again about a week later spending the entire time being very inactive... only this time do not be active on that day and eat the same meal. You will not feel as good about it. The food will sit in your stomach.
If people were more active they could eat pretty much what they want within reason.
Sure. I don't think any of that is in dispute. But that's not what you said. (And as I understand it, there are also more nuances than simply energy density, but that is starting to get into details where I'm fuzzier on how it all works.)
Furthermore, it's also not feasible for a great many people in this day and age to "spend a day being active all day" more than once in a while. I know I work at a desk all day, and a lot of other people do the same. That makes it highly impractical to get the kind of exercise you're talking about. And just increasing exercise a little bit doesn't even always help. Indeed, I moved 3 years ago to be closer to my job, and started walking a mile and a quarter to and from work most days...and my weight didn't even budge.
However, last fall I cut nearly all carbohydrates (simple and complex—that is, sugars and starches) out of my diet, and lost 50 pounds in 4 months. Yes, I was eating a little bit less, but not nearly enough to account for the amount of weight I lost. And I was having bacon and eggs for breakfast many mornings, and plenty of other very rich foods—just not sugary foods or starchy foods. (And it was hard to do. But suffice it to say that I had some strong motivation specifically to cut out carbs for that period of time—the weight loss, in my case, was a nice bonus.)
Now, that's just an anecdote. On its own, it would mean very little. But given that the article we're both commenting on is about a scientific study that shows that I'm not the only one that can be true for, I think it's quite relevant.
Dan Aris