The fundamental reason why you are only fat but not thin is that your metabolism is too slow. If you eat too much, it will turn into fat and accumulate in your body. After a long time, you will become fat. What's more, you exercise too little, don't drink water, stay up late, and lead an irregular life, which also makes you fat but not thin. After the age of 35, the hormone level will gradually become lower and you will get fat.