Why energy expenditure falls faster than weight predicts
Total daily energy expenditure (TDEE) is the sum of resting metabolic rate, the energy cost of digesting food, deliberate exercise, and non-exercise activity such as fidgeting and posture (NEAT). During a sustained calorie deficit, TDEE tends to drop by more than the loss of body mass alone would account for. This extra, beyond-predicted decline is called adaptive thermogenesis: the body becomes more energy efficient and reduces spontaneous movement to defend its stores.
Plotted over time this produces a curve where measured expenditure sinks below the line you would forecast from weight loss alone, then partially flattens. Much of the adaptation comes from reduced NEAT and small reductions in resting rate rather than from large changes in any single tissue.
Real but bounded, and partly reversible
Adaptive thermogenesis is well documented but its magnitude is bounded and varies widely between people; reported group averages are often in the range of a modest percentage of expenditure rather than the very large numbers sometimes claimed. Much of the adaptation reverses when energy intake is restored, though the speed and completeness of recovery differ between individuals. This is general educational information and not personalized medical or dietary advice.