Of course it was built from inside out- what idiot would start with building the outer shell and then haul all the rest of it over the outer shell to dump inside? It would be even more stupid than building a pyramid in the first place. Besides, they did not have concrete pumps yet.
I just always figured they built them by hauling in big stones, fitting and stacking them until they had finished the pyramid. Next mystery?
"Besides, they did not have concrete pumps yet."
Well, maybe the aliens let 'em borrow a couple;)
My research shows that this is where pyramid schemes originated.