In addition to the advantages others listed, I think a big advantage of a large corporation is risk management. If you're a "small" company managing a handful of buildings, and something catastrophic happens to one of those buildings, that could threaten the company's existence. More so if those handful of buildings are in the same city, and all suffer from the same event (flood, hurricane, earthquake, etc.). A large company managing hundreds of buildings spread across the entire country would more likely be able to handle a one-building catastrophe.