[naasking]

Yes, when calories are cut below a certain level, your body downregulates "NEAT" (non-exercise activity thermogenesis), which includes activities like fidgeting, walking around a lot or walking instead of driving, taking stairs instead of elevators, etc. This actually accounts for a considerable number of calories.

Furthermore, a mass M needing energy E is too simplistic because it neglects environmental factors. A mass M in sub-Saharan Africa or a tropical jungle will require considerably more than E for maintenance than the same mass M in a relatively mild climate. This is because your body expends energy to regulate homeostasis (sweating and increased heart rate to cool the body in hot and humid conditions), so more difficult environments require more energy.

That said, "calories in vs. calories out" is still true, you just have to consider that your "calories out" responds to the number of calories in. Drop them too low too quickly, and your NEAT drops to maintain energy balance. But, you can force your body to expend more of that energy by requiring yourself to walk (increasing NEAT), or putting yourself in uncomfortable environments (like saunas), or my forcing yourself into strenuous exercise regimes (weight lifting, HITT, etc).