When things get bigger, they get heavier. But *also* they have more mass far away from the center. So even if a creature is stronger because it’s so big, it takes extra power to rotate one of its limbs around. So that ends up looking slow. That’s why small rodents can scurry but, say, an elephant lopes along.
This specifically has to do with something called moment of inertia, which is resistance to spinning (that is, torque, which causes angular acceleration) in the same way that mass is resistance to moving (that is, force, which causes linear acceleration). Resistance to torque increases much faster than mass. If you double somethings size in all dimensions (width, depth, and height), the mass goes up by eight, but the moment of inertia goes up by 32.
So unless the muscles in your large creature also get stronger on a pound-for-pound basis, it’s going to take longer to just take a step.
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