That is because you are gaining muscle mass. Which is not bad at all. I think BMI should be correlated with the the body fat percentage in order to estimate ho

That is because you are gaining muscle mass. Which is not bad at all. I think BMI should be correlated with the the body fat percentage in order to estimate how many calories one should eat daily (because you are right, you cannot outrun the fork, but you can adjust a bit how many calories you can eat based on how many you burn). And, by extension, have a good indicator of a healthy energy balance.