In many ways it doesn't cancel out. On Mars, things weigh less but their mass is the same. So, for example, if you are walking on Mars at a normal Earth walking speed, and you bump your shin on something, the inertia of your leg as it hits the obstacle is just as great as the inertia of your leg on Earth. In general, a lot of the little physical interactions that you rely on bone strength to get through scale according to mass / inertia (and muscle strength), not weight.