Complex sentence: Schumacher proposed the idea of "smallness within bigness"; in other words, a specific form of decentralization: for a large organization to work it must behave like a related group of small organizations.
Split into simpler sentences: Schumacher proposed the idea of "smallness within bigness": a specific form of decentralization. For a large organization to work, according to Schumacher, it must behave like a related group of small organizations.