I don't think there is anything magical about sugar calories that make you lose weight. I think it is more an issue that protein(meat) calories seem like more of a meal so you end up eating less.
1/2 pound Top sirloin steak has less calories than a large muffin, but yet what kind of meal is a muffin?
Don't think of it as calories, that's merely a representation of what kind of energy is held within a given amount of a given molecular structure.
Sugar, as in, glucose and fructose (and sucrose, which are the two former monosaccharides in a disaccharide form), is fast energy. It takes little digestive energy to get the raw energy out of them, which can be wonderful.
But people don't eat and live like the human body expects. The idea of something rich in sugar or simple carbohydrates is to provide a quick fulfilling burst of energy when your body needs it, or to briefly survive on it until you can find a proper food source.
It has many problems though: it has a tremendous rebound effect due to the movement of glucose/glycogen and insulin throughout the body in response to the consumption and digestion of sugar. The body typically reacts to sugar intake immediately - once the body recognizes sugar in the mouth (and likewise for artificial sweeteners like Sucrolose, Aspartame and Ace K - they fool the body even if in taste they do not), it begins releasing sugar stores because it knows/believes it will soon to replenishing that storage.
That sugar rebound causes a feeling of immense need to eat more to replenish the stores later.
It also has minimal actual use within the body, compared to how much can be easily consumed. We only need so much, but most consume far more than necessary.
Muscles need it for anaerobic energy, and the brain relies entirely upon glucose and nothing else. It doesn't take a large amount, however, to fulfill those needs.
The other part, is protein and fatty acids contain much more energy (so does ethanol

), but they require a little more effort to digest for energy purposes. And they are also used inside the body for just every function outside of muscle movement and brain activity. They are the building blocks of cells.
If you consume excess, or to put it another way, you have a surplus compared to what the body needed that day, proteins and fatty acids can be converted into different products for energy. It's a far more convoluted process that requires more energy and resources from the body - while there is a net gain, it does cost compared to breaking down carbohydrates, even the most complex carbohydrates. However, oligosaccharides and polysaccharides do take more energy to break down into basic energy units than the basic sugars, which typically are already in a basic unit for our body.
Which put it all together, and the body highly prefers to utilize available carbohydrates for energy, regardless of what else you eat or what you prefer. Which means, any surplus fat and protein gets stored, which is the "fat" we know. And as long as you continue to consume enough carbohydates that the body is satiated for energy, you will hold onto that stored fat and probably continue to add more fatty acids and proteins to that fat.
So to answer your question - consuming less sugar (and, in general, carbs/saccharides) often does lead to weight loss that can be maintained if you keep limiting the intake. Why? Because, as you said, proteins and fatty acids are generally slower to digest and thus, more filling (especially because a lot of the better "snacks" that are high in fats and proteins are also good sources of fiber, which is filling yet cannot be digested... or at least some kinds cannot be digested).
Also, if you switch to low sugar, you'll begin forcing the body to start burning the surplus of fatty acids and proteins consumed. More being utilized for growth, repair, and energy = less being stored in the body. Over time, there will be moments where the body does not sense an excess of those being consumed, and decides to finally start burning that stored energy.
If this is kept up, you will probably be consuming less calories on a daily basis (since this diet is more satiating, typically), AND you will be burning more and more of that stored energy, all while storing less extra energy to begin with. Which = weight loss.
In general, you'll also feel a hell of a lot more energized, with less fatigue throughout the day, by switching to a low-carb diet, because you won't be experiencing those massive swings and rebounds. By increasing fat and protein intake, you'll also probably end up having better skin and hair thanks to more fatty acids, of which by the simple odds of consuming enough of the right unsaturated fatty acids and probably more than you had before, you will be improving quite a few aspects of overall health.
Low fat diets took off because the term fat is so misunderstood. Peanut butter is loaded in fat, and almost every single gram of it is wonderful, tasty unsaturated fat.
I always hated labels that show "total fat = 16g" and then go on to only show "saturated fat = 1g trans fat = 0g" and nothing else. What the hell is the other ~15g of fat?! I know it to be polyunsaturated and monounsaturated, but it sure would be nice for the uneducated individual to see these things. Most people have no clue about nutritional science, and I'll readily admit I'm no expert but I try to learn as much as I can to get the nutrients I do need and avoid things which are worthless.
I'll treat myself with sugar and carbs, and get plenty in regular meals. I should cut back, I'm not doing as well as I had awhile ago, and gained that weight back. But I try to keep it smart - oh, I'm starving and I can't think at work, and I'm out of snacks. Well, this bite size candy bar (or two) has enough sugar to perk me up until my next meal. I'm going running later - I can't eat a meal but this cookie has enough energy to satiate me and not send me into zombie mode, energy-starved, mid-run.
A bowl of whole grain mixed-type cereal (oats, wheats, corns, etc) is a slow-digesting meal that should leave me satisfied until lunch, and if I eat the right cereal (though usually more than a single "serving") it usually does. If it doesn't or I didn't even eat enough, I have my "approved" snacks in a drawer at work.
