The volume of a 10cm cube is 10×10×10=1000 cu cms.
The volume of a 5cm cube is 5×5×5=125 cu cms.
Jerry’s mistake was to think that doubling the side length doubles the volume. What he forgot is that the side length is multiplied by itself a couple of times to get the volume, so that means 2×2×2=8. As you can see, the volume of the bigger cube is 8 times the volume of the smaller cube.