Metabolic rates

Image was borrowed. 

Your "metabolism" (the rate at which you burn calories) has three components:

• Basal metabolic rate (BMR) - energy you need to live. Breathing, pumping blood, maintaining body temperature. It uses up about 50-60% of calories.
• Thermic effect of food (TEF) - energy you use around food. Eating, digesting, and storing. 10-15%
• Activity-related energy expenditure. All the getting around and exercising. Rest of the stuff. 

If you want to lose weight, easiest to fiddle with the third one. Mighty hard to maintain weight loss by fudging the first two, which are determined by various factors, such as genes, gender, body composition, activities, and so on. As you lose weight, your BMR goes down as well. As you eat less, so does the TEF. So it gets harder and harder to lose weight.

Fidgeters have higher BMR, so they tend to be leaner. But fidgeting is genetically determined, so if you weren't born one, it's hard to pick up the "habit" (it's really unconscious movement, so it's not like the fidgeter can choose to start fidgeting).