Brown fat is located near the top of the back and its activation can help with thermogenesis in cold weather (helping us to stay warm), and it can help with glucose regulation and general metabolism. It can be activated with exposure to mild cold for a couple of hours per day.
Well worth doing a few Google searches on brown fat. It’s kinda like passive activation of some of the benefits of physical exercise.
Your weight isn’t as controlled by calorie intake as you might think. One time during the wrestling season my calorie intake was small enough and my activity level was high enough that I should have been able to drop my weight as low as I wanted. But that didn’t happen. My weight dropped to a certain level and stayed there. I had to lose the rest of the weight by dehydrating. I’ve also gained weight at a rate that couldn’t have been accounted for by excess calories. I gained 60 lbs. in 120 days when they put me on Zyprexa. That would require a 1,725 calorie excess a day, if you tried to explain that weight gain by calorie intake alone. I think fluid levels in your body affect your weight a lot. Also, when your operating at a calorie deficit your metabolism slows down enough to maintain your weight at a certain level.