You are wrong. Mashing a big gear stresses the cranks and bottom bracket a lot, but mashing a little gear confers mechanical advantage and so by the time the force is transferred to the dropouts, it winds up being greater (other things being equal). This is why you can accelerate faster in a low gear. I think there's more to the explanation than this, but that, I hope, is the basic truth. Newtonian physics as applied to bikes is actually a bit tricky for me to grasp.