All activities and movements are done by muscles not by fat. Every body even the endomorphic can increase muscle mass by doing gym exercises and consuming small multiple 6-8 balanced meals. Exercise will help you to stimulate the muscle to get bigger and stronger and multiple meals will help to provide continuous nutrition to recover and build muscle muscles and not leaving surplus calories to get converted to fats. So let your meals be small and balanced and exercise be moderately heavy and short. Take sufficient rest between sets and training days. Avoid over eating and over training.