> The stresses in the dome were found to be substantially reduced by the use of successively less dense aggregate stones, such as small pots or pieces of pumice, in higher layers of the dome. Mark and Hutchison estimated that, if normal weight concrete had been used throughout, the stresses in the arch would have been some 80% greater. Hidden chambers engineered within the rotunda form a sophisticated structural system.[55] This reduced the weight of the roof, as did the oculus eliminating the apex.
That said, I'm guessing the Pantheon is in one piece (unlike the coliseum) because it's been used as a church for the past ~1600 years and presumably (similar to Notre Dame) it gets repair as-needed.
It is not scalable, but the quality is quite stable across batches