What component of physical fitness refers to the proportion of fat and muscle mass in your body, and will change according to your eating and exercise habits?

BMI is fat percentage based on height and weight but excludes muscle mass and may indicate a person is obese when they actually are not.  You may need to get some body fat calipers and take some measurements of your own and then do a little bit of reading online to see how people of your "body-type" should determine results of a BMI test.