Because it doesn't appear to either be designed at a consistent scale or be a solid rectangular cuboid making volume feel like an even less accurate basis for my silly joke.
The original took 144 years to build, and this model is anywhere between 153 to 278 times smaller. If one were to scale by volume, the build time would take at most 22 minutes, and at least 3.
And I think for the actual value you get, scaling by volume would actually be more accurate.