Megatron is smaller than Optimus Prime. DotM Voyager Megatron is kinda "Motormaster-esque" in that the robot incorporates both the cab and the trailer all in one (the cab forms the upper half of the robot, while the trailer forms the lower half), so the result is a smaller scaled truck mode. Optimus Prime is only the truck-cab, so the vehicle is a larger scale. I can post a side-by-side comparison pic later if you like.