Well let me tell you this. I have gained 20 lbs of muscle in the last year, and I look exactly the same. Muscle is much denser than fat, so you can actually put on a lot of muscle, while staying the same size, or even shrinking.

At the very least, I would encourage some cardio because it's the best thing you can do for your overall health, and it can also help if you need to run from the cops.