A plague upon scales!
Remember that weight is just one of many ways to measure your progress, and on its own can be very misleading. As you rightly say, muscle weighs more. Bet you feel fitter and healthier right? Let that be your guide and avoid the scales for a while.
Keep up the good work
Its muslce, and its natural. Some oeople just gain muscle better than most. If you want to decrease muscle mass, start doing long distance cardio or ristance training