The only time your body changes and gets leaner or bigger is when it has to. The body is a bio-machine designed to adjust and optimize itself for survival so every single thing you do influences that change. If the conditions dictate it, it will get stronger, bigger and more durable simply because it makes it easier for it to cope with the load. For your body it’s all about conserving energy - it’s easier for you to move stones, pull yourself up cliffs and chop down trees if you have the arms for it.