Yes, your ribcage can become smaller when you lose weight. This occurs due to losing subcutaneous fat, the layer of fat directly below the skin. Weight gain results from consuming more calories than you use in activity. The body stores excess calories in fat deposits and this affects your circumference at the ribcage. Although there are variations in body fat distribution, most people lose upper body fat before lower body fat.
Most areas of your body, including hips, torso and even feet and fingers, can become smaller when you lose weight. Bones and body frame size don't change with weight loss, so the ribcage itself won't change in size. As you lose overall body fat through weight loss, your measurements decrease. Many people store body fat on their backs and throughout the torso. As this layer of fat over your ribcage and back decreases, your body's size at the ribcage becomes smaller. Women may reduce their bra band size and both sexes will fit into smaller clothing as weight loss progresses.
Body fat includes subcutaneous fat and abdominal fat. In some people, particularly men and women with the "apple" shape, body fat distribution includes a significant amount of abdominal fat. Abdominal fat often increases with age, due to age-related muscle loss. Inactivity can accelerate this process. Abdominal fat increases the size of your torso and ribcage. Deep abdominal fat known as visceral fat covers your internal organs and increases your risk of heart disease and diabetes. Exercise is the most effective way to burn off all forms of body fat, including both forms of abdominal fat. Combining exercise with a healthy moderate calorie-restricted diet can help you reach a smaller ribcage size.
Exercise helps to make your torso smaller at your ribcage by improving your metabolism and making your body firmer. Exercise, especially weight-bearing exercise and strength training, also increases bone strength -- helping to prevent the risk of osteoporosis and fractures. Men who lift weights may not develop a smaller ribcage size during weight loss because men who build muscle can increase the width of the torso and create thicker back muscles, creating the popular masculine V-taper. In this case, the body develops a healthier ratio of muscle to fat and will have a smaller waist size. For physical fitness and weight loss, perform at least 30 minutes of cardio five days a week and at least two 20-minute strength training sessions each week.
Women who lift weights become firmer, but don't create muscular bulk unless they apply focused training toward that goal. Because muscle is more compact than body fat, women will become stronger and firmer from strength training.
Cut 500 to 1,000 calories a day from your maintenance level, along with exercising, for a safe rate of weight loss of 1 to 2 lbs. a week. Don't reduce calories below 1,200 for women and 1,800 for men.