An economy running on muscle power has a hard and low ceiling on total production, because its only real input is land and the returns to more of it fall away fast.

New land can be brought under cultivation (or cultivated more intensively) of course, but marginal gains decrease rapidly (because the best land is cultivated first and because adding more labor to already cultivated land, while it can increase harvests, is less efficient than cultivating new land) and there is a fairly hard ceiling on total production of this sort that was, for the most part, fairly low.