Excluding massive biological changes (we're not going to start carrying more water or drastically increasing bone density), you gain weight by adding muscle or

Excluding massive biological changes (we're not going to start carrying more water or drastically increasing bone density), you gain weight by adding muscle or fat. Adding muscle is generally healthy (not in the case of heart disease, organ growth from steroid abuse, etc.) and also very hard. Adding fat is very easy. It would incredible if our population was adding a substantial amount of lean body mass. Are there any other causes you could think of that would cause a BMI increase across a sufficiently large data set that would not be caused by fat?