140 rear is better here. The smaller rotor is lockup control on application and control on sustained braking. For sustained braking the size rear rotor doesn’t really matter because to transfer the weight to the front. On dry dirt with a big soft tire, you often have more traction than asphalt, so you can get away with more brakes, but the smaller brake saves your rear tire and let’s you control the rear wheel more. I’m much heavier and could do a 140.