"Ahu" are stone platforms on which many moai sit. There are 313 known ahu and 125 of these carry moai. The biggest, Ahu Tongariki is 220 m (720 ft), and had the most (15) and tallest moai.
At some point, you can't lift this boulder with just your own strength. And if you find that you need to move bigger and bigger boulders up hills, you will need more and more help.