Actually, a company doesn't need to have *monopoly* market power to be too big and powerful. If it is part of an oligopoly (a small number of companies that together control a market), it and its co-oligopolists are too large and powerful.

I believe it was John D. Rockefeller's Standard Oil that was actually organized as a series of "trusts" which owned majority interests in dozens of supposedly competing oil companies and coordinated their activities in ways that exercised market control despite being a diffused group of companies.