They have lower metabolisms. Their bodies are less efficient at burning calories and more efficient at storing them. Fit people burn up extra calories before they store any as fat in the body. Overweight people store extra calories as fat. Exercise changes metabolism.
Regular exercise allows fat people to lose fat and control their weight.
The most effective workout program includes both aerobic exercise and strength training. The combination yields faster fat loss at any age as your body burns fat for fuel. Aerobic exercise increases metabolism. Strength training, or weight training, builds muscle. Your heart rate, speed, strength, and metabolism all improve as you get fitter and trimmer.