Stephan, the muscles get large in our legs because we stress them every time we walk or go up stairs. Our arms don't get stressed as much unless you have a job where you do a lot of lifting. So with MC it is fairly common to have much larger lower body muscle groups (calves & thighs).
When you go to the gym, it's best to start with lighter weights, fewer reps and fewer sets. If you feel burning, then your damaging the muscles. While bodybuilders push to that point, for us it's not a good idea. It's better to build up slower because we don't recover from the damage as well.
Sometimes there is a rebound effect where you are very stiff, and then very weak. It has to do with the shift in potassium. Many of us find that drinking skim milk before and after a workout helps because it contains all the electrolytes and nourishes the muscle cells.