Basal metabolism is the energy requirement of doing nothing. It is the metabolism that must be maintained at all times to stay alive. It is the rate at which you burn calories when you are a complete rest in a comfortable environment several hours after eating anything. Basal metabolism maintains the processes and integrity and readiness of your body. It keeps your brain doing whatever it does.
For most of us who aren't running in a marathon or racing in the Tour de France the basal metabolism is where most of our food goes. It is increased by healing from burns or surgery, pregnancy, fever and most kinds of physical stress. It is decreased by dieting, starvation, malnutrition, fasting and increased age.
Basal metabolism correlates with your size or more exactly with the size of your lean body mass. Lean body mass is the fat free part. Age and sex and surface area factor into determining your basal metabolism. There is also some familial tendency. Your basal metabolism is like your parents' or brother's. After all these things are considered there is still some individual variation.
For the purpose of comparison we might consider the average 150 lb, 40 year old husband. His basal metabolism plus the energy he needs to eat and process food plus a minimum amount of physical activity for a desk job will be about 2000 calories a day.
If you take obese people as a group and compare them with thin people, they have the same average basal metabolism. Actually obese people have an increased basal metabolism because they have more lean body mass together with the extra fat. And they have more surface area, an important determinant of basal metabolism. But when we correct for the extra lean body mass and surface area they have the same basal metabolism as thin people pound for pound.
So, big people burn more calories than thin people. This doesn't seem to be doing them much good. Basal metabolism is not their problem.