Ah so maybe the Garmin calculates the basic at rest calorie burn. Think my Apple Watch calculates 'active calories' - so only calories burned above those the body uses to maintain itself at rest. So my daily calorie average is about 1000-1200 with a decent 1hr workout at the gym plus 10k steps etc.
The Garmin also does active calories too although I accept that I need to up these (and 2 hour hilly walk today was a decent enough contribution) while avoiding using Belgian sports drink to rehydrate afterwards.
A 180cm 100kg lightly active man needs 2700 calories to maintain body weight, so 3000 calories for a large person isn’t that far off.