Eating anything in excess will likely cause weight gain.

I've yet to see a study that says you can eat more calories than your metabolism burns and you can lose weight, no matter what the food. Now there are numerous examples saying one macronutrient ratio produces greater amounts of weight loss than another when isocaloric (a constant level of calories). Albeit, it usually is quite small and often statistically insignificant in the time span of the study itself.

The advantage of a high-protein-low-carb diet is rather small, we're talking maybe 50-100 calories per day. It's certainly not 100s to 1000s of calories of advantage.

Our body, like everything in the universe, follows the laws of thermodynamics. In order to lose weight you do have to consume less energy than you spend.

You can consume 3000 calories of the most healthy foods available, if you daily caloric expenditure is 2800 calories you will gain weight.

I think it's the average of all the above.

1. Calories in/calories out is true but irrelevant. Some people dispense with the extra calories they overheat by fidgiting and so don't gain. Some people can cut calories and be energetic, and therefore lose weight, whereas others will just become exhausted, sedentary, and therefore burn less and not lose weight. Gary Taubes sites examples of populations that eat under 2000 cal/day, work hard, and are obese (they live of bread and coffee). They're also completely exhausted the rest of the time. Food composition matters. Individual genetics matter.

2. Food consumption matters. We all know that when somebody crash diets, they lose weight. True, it might be more muscle than fat in some cases, but often people look thin after a long crash diet. It's not sustainable, sure, but it demonstrates that calories matter.

I think the right way to think of it is that, for most people, cutting carbs and eating paleo triggers satiety with fewer calories than one would consume otherwise, and also that eating the same number of calories may still result in better weight control for a paleo, who will feel more energetic and so expend more calories than they otherwise would have. It's a combination.

Anicdotally, I occasionally do go on a month-long paleo "binge" where I overeat paleo foods. Sure I gain weight during those times. I don't gain as much as I used to, and merely getting my portion sizes back in line gets me back where I want to be, a gigantic change from my previous life. My health and weight are vastly improved, but at least for me I still have to keep an eye on whether I'm eating for hunger or for entertainment, because eating for entertainment, even with paleo, is not healthful for me.

Of course it's possible if you're eating 3000 and burning say, 4500. The "all calories are not created equal" has more to do with how various foods affect you metabolically, and how they will thereby influence body composition.

FOr instance, 2 cups of broccoli (food A) has x number of calories but has virtually no insulin response to eating it and takes 2y calories to digest. A slice of light wheat toast (food B) has x calories too, but takes y calories to digest an ilicits a large insulin response. Therefore x calories from broccoli is the better option if you are trying to lose body fat when choosing between x number of calories from food A and x number of calories from food B.

I think comparing 2000 cals meat and veggies to 2000 cals chocolate is meaningless. I was going to say you couldn't survive on 2000 cals/day chocolate as it contains only trace protein, but milk chocolate is 17% protein according to FitDay! Still, to compare apples to apples, since a certain amount of protein is required for "maintenance" of cells as is a small amount of fats that are also components of lipoprotein particles and rarely incorporated into fat stores. These requirements are fairly constant if you've not changed activity/environment.

So, lets compare 2000 calories of intake in excess of those needs. Now I suppose we could compare whether those 2000 calories are from meat & veggies or chocolate (and for the sake of argument let's use conventional milk chocolate). Well, if one presumes the meat and veggie diet is 60% fat, 30% protein, 10% carb = 1791 using average Jequier thermogenic factors, while milk chocolate is 24% fat, 17% protein, 59% carb = 1812 effective calories. Pretty much a calorie is a calorie for this comparison, though the thermogenic factors may already be accounted for in the average calorie estimates.

Studies have shown that you absorb fewer calories from less processed foods, and that includes a whole steak vs. ground beef. Thus the types of foods are more important than the macro composition.

A milk chocolate diet! LOL Sounds good :D

There is a great explanation of this in the Book the Smarter Science of Slim. It is not a Paleo book. However the Paleo aware will find it very Paleo friendly. Just remember that Soy is bad for you while reading the book. I also would not eat the amount of flax seed he is eating. Other than that it is very Paleo Friendly.

The perfect Health diet has some information on this as well.