“Researchers Made 3,900-Pound Boulders They Can Move by Hand, Giving More Insights Into Ancient Engineering”

By using concrete of different densities and some careful design, researchers were able to make quite large and quite heavy (up to 1,770kg or 3,900lb) monoliths which one or two adults could relatively easily move around and use to assemble larger structures.

https://gizmodo.com/researchers-made-25-ton-boulders-they-can-move-by-hand-1834106230