Its easier to understand if you ever see those cross sections of cadavers, but when you gain body fat, its not just pushing out, its also pushing in, and it adds weight to you. This puts pressure on your organs and your muscles and bones(lungs and heart included) to keep up and support you. Because obesity is often caused by poor eating habits and/or lack of exercise, and your body is now storing a large excess of fat, it tends to go hand in hand with high cholesterol(which can lead to blood clots and heart disease) and high blood sugar(which can lead to diabetes).