The simple answer to this is that some peoples body requires more energy (faster metabolism) than others. For example, you and your friend are of the same age and weight but she can enjoy a scoop of ice cream every day without gaining any weight, while when you do the same, it may lead to weight gain.
Another factor responsible for the same can be the resting metabolic rate (RMR). RMR is the number of calories that your body uses while you sleep, breathe or just sit. And you will be surprised to know that RMR accounts for 65 70 per cent of calories that you burn in a day.
RMR varies from person to person. How much lean muscle mass you have plays a major role in deciding your RMR. Other factors are genetic, age and size.
See the article here:
Explained: What is metabolism and all the buzz around it - Times of India
