F = m * a
Look it's right there, force equals *mass* times acceleration. On earth, Scarecrow is 342kg * 1g when stationary. On mars Curiosity is 900kg * 1mars-g *when stationary*. Sure the vertical force on the tires is the same when standing still, but what about the force required to stop 342kg vs 900kg of inertia if you hit a large pointy rock at 1m/s?. In this case, with the same initial velocity, the acceleration would be the same but the force experienced by Curiosity's tires would be ~3x larger (ignoring any shock absorption).