an extra 10 pounds of muscle, which is average for someone who has never trained, maybe burns only 100 extra calories. The problem also is that muscle hypertrophy also entails gaining some fat too.
You might need to put on some fat to gain muscle EFFICIENTLY, but it's not a requirement for putting muscle on at all.
But my point is, if someone loses a bunch of fat their bmr is going to take a hit. Could be pretty significant on the order of hundreds of calories a day.
Read that again. Six kilograms, per year. Every year.
Given a person consuming near their total daily energy expenditure, that’s the difference between losing almost 20 kg since the start of COVID, and gaining 20 kg.
Small changes over time compound in unintuitive ways, which is all the more reason not to dismiss them so flippantly.