Past a certain size, growing bigger starts making a company less efficient, not more

Economies of scale let bigger firms produce each unit more cheaply, up to a point. Beyond that point, diseconomies of scale kick in: communication breaks down across more layers of management, coordination costs balloon, and individual workers feel less accountable in a sprawling organization, so the average cost of each unit produced starts rising again as the firm keeps growing. It's a major reason why companies that dominate a market can still lose ground to smaller, leaner rivals.

— Wikipedia contributors, Diseconomies of scale — Microeconomic cost theory